@ToozeDE Hello! That sounds super frustrating and I'm sorry you're having this problem. I had not encountered this error so I did some research. It seems that error came up quite a bit with some apartments in City Living and it was caused by stairs. So, if you have any stairs on your lot, I recommend moving them out or deleting them. Then try to add the roofing and then replace the stairs.
In general, I've had a lot of problems editing Residential Rental lots. It is especially bad when the lot is owned and/or sims are living on the lot. With that in mind and if the stairs suggestion doesn't work, I suggest editing the lot as a Residential instead. This means moving all sims out, selling the lot, changing the lot type to Residential and making your edits. Then you can set the lot type back to Residential Rental, have your sim buy the lot and move the tenants back in. It's not ideal but I've had much more success with editing lots this way.
Please come back and update if any of this works for you or not.