How do I remove these floor tiles?
I downloaded a lot made by DAONSIMSLIFE. There are three (3) floor tiles floating in the air and I can't delete them. As far as I can tell the tiles aren't part of a roof for the platform underneath.

No, not yet. I decided to just put a roof over the platform that holds the urns.
I have no way of notifying the creator so this will have to do for now.

Very likely that's what was there originally. A room, that would create ceiling. Removing walls one at a time does not usually remove the ceiling/floor that generated. Only highlighting room and pressing delete will get rid of everything in one go.

Typically, though, using sledgehammer tool on the remaining, floating ceiling/floor tiles should be enough to get rid of them. Failing that, if it's somehow stuck, I would rebuild a room below, click on room to see yellow guideline above it and click on yellow guideline to command game to rebuild ceiling. Then you should be able to delete rebuilt ceiling properly. Then remove walls one by one. Don't instant delete room and ceiling, otherwise it will also delete that platform and urns I'm sure you want to keep. (Exactly the method the original builder used so as not to delete platform and urns, but missed removing floating ceiling tiles afterwards.)

Though, to be honest, I would have imagined that platform and urns was always meant to have a roof, and four wooden pillars to hold it up, hence why the floating ceiling tiles are there at all. That seems more natural to the design than a raised platform full of urns exposed to the elements with no roof to protect from rains and snows and passing seagulls crapping on your beloved dead.

I tried all of that but thank you anyways for the suggestions. I opted to just build a roof over the platform. It seemed to be the easiest fix.