I found a few things that sound positive that you might try if you have not.
1. Assign Rooms to Units: Ensure every room is assigned to a specific unit (e.g., Apartment 101, Apartment 102).
2. Adjust Lot Type:
Try changing the lot type to "Residential" temporarily, make your desired changes, and then switch back to "Residential Rental".
3. Use Free Build Cheat:
Enable cheats by pressing Ctrl+Shift+C and typing "testingcheats true" then "bb.enablefreebuild" to bypass restrictions and build anywhere.
I struggle with building messages like that too periodically so I understand your frustration. I hope you get it sorted out!