Hello everyone!! sorry if I've posted in the wrong place or there is an obvious answer to this, but I haven't been able to find one yet unfortunately

So I've been trying to convert one of those lots in Vetonaville that's 2 houses on one lot into 2 apartments, and after putting everything together every time I try to change the lot zoning the game tells me that there is an invalid apartment on the lot and it needs to be fixed... But I can't figure out what's wrong!! It is completely fenced in in the back, the front doors are apartment doors with the mat facing outside, I've tried deleting & replacing all the windows, doors, fences, fireplaces, and pillars, yet nothing works... does anyone have any idea?? I've attached some images in case there's something obvious i'm missing.

Thanks....

If you build it on a foundation, you have to put floor dividers around the back door on top of the foundation, so that the back yard is considered common area.

Aaah that fixed it!!! Thank you so much!!! <3