Found solution for create unit not working
Through trial and error I figured out how to be able to create more units for The Sims 4 For Rent.
Removing mods, deleting localthumbnailcache, repairing the game didn't work. What worked for me removing all tenants, selling the property and rights for rental, and then finally flipping the lot type from residential rental, to residential, then back to residential rental. After that I was able to create more than one or 3 units.
Leaving this temporary fix for anyone else that ran into the same issue. Initially I believe it must've been a mod that dealt with UI; but I did 50/50 for so long and that didn't help.
To describe the bug a bit more: When you'd try to create a unit (prior to 6 unit limit) the game would instead recolor the room to an already existing unit and not make a new one.
If this happened to you try the solution I provided above.