I know this is like 2 years later but still the only place I've seen this mentioned and today I think I found a solution.
I had to enable free build mode using cheats first, then go into build mode to see which apartment I was supposed to enter (it was the only one that the interior loaded for in build mode). After that I went back to playing my sim and the knock on door option popped up. I'm guessing the apartment didn't load correctly or something went wrong with the game knowing which one I was supposed to get access to.
