Edit: Thank you @Nettlejuice for pointing out that if you can’t place an apartment from the gallery that it is probably due to this issue as well. You have to bring the apartment back in to your livable space first before you can place the apartment.
If you have attempted to remodel or rebuild these apartments then you will have noticed that sometimes a glitch occurs once you use the bulldoze function (or individually knocking down walls). A section of the apartment is no longer part of your living space although it is still contained within your walls. You cannot place flooring.
Until a patch arrives, the work around is to create a giant room and extend it throughout the entire apartment. This brings the section of that apartment back in to the the editable living space.
Screens below with the first screen showing the darkened area that you are unable to modify till you create a giant room enclosing that area.