Got the same problem. It seems like when we divide a lot into apartments, for some reason the first apartment has a "home" checkbox, although no one lives in the house, and that apartment is automatically designated as the owner's apartment. But there is in fact no owner, we can buy and sell this lot.
Here's what I did to fix this in my game: I assigned all the rooms to one apartment, made the lot residential, destroyed it completely, placed a random Sim on this empty lot, put my house from the library before dividing it into apartments, made it an apartment, divided it again for apartments and this random Sim, who is also the owner, now lives in one of the apartments. For now it goes smoothly, but I'll keep the old save file to rollback everything if it happens again.
I had the same after I tried to buy this rental lot with another sim and even after he sold it I still couldn't do anything. That time I loaded earlier save which was before that sim bought a lot and then did what I described earlier
