I have finally learned how to create body presets in blender and so I downloaded the sims 4 morph maker app so that I can create d maps etc.. but I keep getting the message that "the Index was outside the bounds of the array." I am lost as to what that means so I've been searching everywhere with no luck. I already tried downloading a new blender and I made sure to check the important properties in blender before exporting. But still nothing, someone please help!

It probably means you did something in creating the morph mesh that changed the number or order of vertices and/or faces, or that Blender changed them, or that you exported the morph mesh without normals. Make sure you don't change anything except to move vertices, and that you're using the correct settings when you export the mesh from Blender.

If you upload your base and morph meshes here I'll take a look.

Omg thanks for the reply but I've finally gotten passed it, I had to re export the base body from morph maker and it worked but now when I preview my morphed body it looks glitched (Looks like the parts that I didn't change are stretched etc..), also how can I send it because it says the file type isn't supported. This is my first time on here so I don't know how anything works. At least here is my screenshot of the body in mm.

That's some pretty impressive distortion! To upload the base and morph meshes, you have to put them in a zip archive. The easiest tool to use IMO is 7-zip: https://www.7-zip.org/

Ok here you go, Thanks sm for your help btw.

Your morphed mesh is positioned below and in front of the base mesh. That's what's causing the distortion - the data in the DMap is the difference between the original and moved positions, and DMaps can't handle large vertical movements. I moved it to the correct position and the morph came out fine.

Sorry to keep bothering you but what do you mean you moved the position? You moved it in blender?? I don't see what you see?

UPDATE:
I fixed it, thanks sm lmao

One more thing, I am done with everything but it's not showing up in game and yes i already deleted the localthumbcache and restarted the game.

UPDATE AGAIN; Nevermind got it lol

Ok one more thing that I can't fix is the thumbnail, I tried changing it twice and the game keeps loading it as their own picture, what should I do?

Please use edit post function so you are not creating so many posts in such a short span of time (moderators don't like that).

Delete localthumbcache.package in your Documents / The Sims 4 folder.