Yeah, I don't really know what happened.
I started with a long rambly part because I have AuDHD and never know how much information other people need, particularly in a troubleshooting context for things I don't know how to fix - I know a pretty decent amount about computers but some things entirely escape me.
Backstory time. Can possibly be skipped in favour of the current SimPE problem at the bottom.

The first thing that started happening was that I kept getting an ACR error (I play with testingcheats on) about how a subject needs to be a person... I had the usual three options of Reset, Delete, Cancel. Cancel didn't do anything and the error would come back. Same with Reset. So I eventually just hit 'Delete'.
It went away temporarily, but came back every time I went to a downtown lot.
The only thing I can remember is that the only recognizable thing that would sometimes be in the error message (sometimes it was words that made no sense to me or no error message, just a log report number) was the name "Denise".
More errors kept popping up in other households, even at home this time, whenever my Sims would try to roll a new want/fear it would give me an error that I knew was related to the other ACR one. Eventually, another error gave me something else, which was the character ID, from which I used SimPE to deduce that it was Denise Jacquet causing errors. I've never even opened their household so it must be from community interactions that she has any tokens or anything particularly ACR related.
Her Turn Ons and Turn Offs were completely blank in SimPE but I don't recall anything else particularly strange.
Here's where I probably messed up.
I had to reset my OS on my computer (long story) around Jan 2 so I lost my programs but not my files. When I reinstalled Sims 2 Legacy, I still had all my Downloads and such, so at the time all I had to do was reinstall CEP (since it was a 'program') and I was good to go.
Then... I got the bright idea of "oh, if it's gonna keep throwing that error message, maybe I'll go see what's up with Denise's file in SimPE now that everything is fresh anyway"; and SimPE wouldn't really load properly, probably because of the reset, but I couldn't uninstall it since it didn't register as a 'program', so I just deleted the files I could find and used the "search" feature in my file explorer to find the rest, and re-downloaded it, and re-installed it, and thought that would be fine.
Turns out I had SimPE installed in two locations (in \documents\the sims 2 legacy\downloads and in users\[user]\downloads) that whole time, and I didn't get rid of every bit, so things were... not loading right?
But then I also -- incredibly stupidly, I know, incredibly incredibly stupidly -- got the even brighter idea of "oh wait, I made a backup of my files anyways, I'll just put Denise's character file in particular and put in the old one since she doesn't know any of the sims that have happened since the reset anyways and there won't be any corruption or missing tokens or whatever".
Hoooooooly macaroni I was wrong. The game did a big mad. First time I tried to load it, Denise was the ONLY Sim in the entire neighbourhood, which made me realize I had accidentally put her directly into \N002 instead of \N002\Characters which was also my idiocy, but then when I put her in the right folder, I tried to reopen my game and... all my sims are back, but they're all A. completely in the red for aspiration, and B. the wrong aspiration (most of them are Grow Up or Grilled Cheese?). I haven't opened any of the families because I can see that just from the hood.
So uh... yeah. Those ones were entirely my stupid mistakes.
So I've been looking into it in my spare time for three days trying to figure out if I can fix it or if I need to either use the entire backup or a completely new clean Strangetown, because I'm three generations in on some of these families (Nervous Subject's eldest child just had a kid) and I don't want to completely wipe it. I played a lot of the game between the last backup that I can most guarantee is clean (Dec 28) and the neighbourhood that's, at this point, probably corrupted (Jan 4).

Long rambly part over!
Because uh...... I've now run into another problem that I actually don't know how to fix? I don't know what I did between the last time I opened it and now -- genuinely -- but SimPE just... won't open/load properly anymore as of today.

I have gotten several different errors while trying to do several different things, like "access to the path is denied" (I went in and realized that a bunch of the items in the current install location were Read Only so I fixed that one), but the one I am very stuck on is "unable to start tool plugin". I can't seem to find a good way to check out what the hell I did this time, because all of the solutions I can find either aren't working or require things I don't really know how to do.

Here's the details it threw me:

Message:
Unable to Start ToolPlugin. SimPE Version:
Default (0.75.1.31194).

Exception Stack:
System.Configuration.ConfigurationErrorsException: Configuration system failed to initialize ---> System.Configuration.ConfigurationErrorsException: Unrecognized configuration section system.serviceModel. (C:\Windows\Microsoft.NET\Framework\v2.0.50727\Config\machine.config line 146)
at System.Configuration.ConfigurationSchemaErrors.ThrowIfErrors(Boolean ignoreLocal)
at System.Configuration.BaseConfigurationRecord.ThrowIfParseErrors(ConfigurationSchemaErrors schemaErrors)
at System.Configuration.BaseConfigurationRecord.ThrowIfInitErrors()
at System.Configuration.ClientConfigurationSystem.EnsureInit(String configKey)
--- End of inner exception stack trace ---
at System.Configuration.ClientConfigurationSystem.EnsureInit(String configKey)
at System.Configuration.ClientConfigurationSystem.PrepareClientConfigSystem(String sectionName)
at System.Configuration.ClientConfigurationSystem.System.Configuration.Internal.IInternalConfigSystem.GetSection(String sectionName)
at System.Configuration.ConfigurationManager.GetSection(String sectionName)
at System.Configuration.PrivilegedConfigurationManager.GetSection(String sectionName)
at System.Diagnostics.DiagnosticsConfiguration.GetConfigSection()
at System.Diagnostics.DiagnosticsConfiguration.Initialize()
at System.Diagnostics.DiagnosticsConfiguration.get_IndentSize()
at System.Diagnostics.TraceInternal.InitializeSettings()
at System.Diagnostics.TraceInternal.WriteLine(String message)
at System.Diagnostics.Trace.WriteLine(String message)
at SimPe.WaitingScreen.get_Screen()
at SimPe.WaitingScreen.Wait()
at SimPe.Plugin.NeighborhoodForm.UpdateList()
at SimPe.Plugin.NeighborhoodForm.Execute(IPackageFile& package, IProviderRegistry prov)
at SimPe.Plugin.NeighborhoodTool.ShowDialog(IPackedFileDescriptor& pfd, IPackageFile& package)
at SimPe.ToolMenuItemExt.ClickItem(Object sender, EventArgs e)
System.Configuration.ConfigurationErrorsException: Unrecognized configuration section system.serviceModel. (C:\Windows\Microsoft.NET\Framework\v2.0.50727\Config\machine.config line 146)
at System.Configuration.ConfigurationSchemaErrors.ThrowIfErrors(Boolean ignoreLocal)
at System.Configuration.BaseConfigurationRecord.ThrowIfParseErrors(ConfigurationSchemaErrors schemaErrors)
at System.Configuration.BaseConfigurationRecord.ThrowIfInitErrors()
at System.Configuration.ClientConfigurationSystem.EnsureInit(String configKey)

Source:
System.Configuration

Execution Stack:
at System.Configuration.ClientConfigurationSystem.EnsureInit(String configKey)
at System.Configuration.ClientConfigurationSystem.PrepareClientConfigSystem(String sectionName)
at System.Configuration.ClientConfigurationSystem.System.Configuration.Internal.IInternalConfigSystem.GetSection(String sectionName)
at System.Configuration.ConfigurationManager.GetSection(String sectionName)
at System.Configuration.PrivilegedConfigurationManager.GetSection(String sectionName)
at System.Diagnostics.DiagnosticsConfiguration.GetConfigSection()
at System.Diagnostics.DiagnosticsConfiguration.Initialize()
at System.Diagnostics.DiagnosticsConfiguration.get_IndentSize()
at System.Diagnostics.TraceInternal.InitializeSettings()
at System.Diagnostics.TraceInternal.WriteLine(String message)
at System.Diagnostics.Trace.WriteLine(String message)
at SimPe.WaitingScreen.get_Screen()
at SimPe.WaitingScreen.Wait()
at SimPe.Plugin.NeighborhoodForm.UpdateList()
at SimPe.Plugin.NeighborhoodForm.Execute(IPackageFile& package, IProviderRegistry prov)
at SimPe.Plugin.NeighborhoodTool.ShowDialog(IPackedFileDescriptor& pfd, IPackageFile& package)
at SimPe.ToolMenuItemExt.ClickItem(Object sender, EventArgs e)

Windows Version:
Vista

.NET Version:
2.0.50727.9179

I'm going to be real. I have no idea what most of that says contextually. I've done some computer programming, I know some HTML and Java, I like to mess around with settings a lot on things that I am confident on just to see what I can do, but I have never looked into the inner workings of SimPE and such so I don't know what it's asking or telling me, or what I should do next.
(also I don't know why it says Windows Version: Vista when I'm running Windows 11 but we can probably ignore that right?)
It's the SimPE version that I saw some folks recommend as being the best for the Legacy version, which is what I'm currently playing (I owned the whole thing for years but I find Legacy runs smoother so I've yet to run or reinstall the disc version since Legacy came out), which is SimPE Version 0.75.1.31194... if I should be using a different version please let me know, people who know more than I do

Restore the neighborhood from backup. You messed up with the part where you moved the character file into the neighborhood folder.

As for your ACR errors- based on your description of the problem, it sounds like Denise Jacquet's aspiration got un-set. It would be a matter of using the ReNuYuSenso Orb to give her an aspiration or various objects such as Sim Modder, Sim Blender, insimenator, etc. to reassign her aspiration. I can't say for sure without you posting the actual error log though.

Denise Jacquet in particular also has a pre-existing issue with her dead husband that can cause errors like this. You don't need SimPE to fix it, though, just use the Batbox to Fix Dead Romances.