I have a feeling I am missing something really obvious - I can't figure out how to get the triangle to enable so I can save the room. I've removed/replaced all the items. I've built the room multiple times - multiple places - larger sizes (everything I can think of). No matter what I do I can't get the triangle to enable. What am I doing wrong? Thanks.

The triangle doesn't go green like the little tick mark, it stays grey, but it should have a blinking little circle around it which means the room can be completed. It always takes several button bashes of the triangle before the room will complete/finish for me on PS5 though. You've probably done this, but have you tried pressing it several times, not just once or twice?

Also, make sure there isn't an item selected or you're not still in the menu, because it doesn't seem to finish for that either (although from your screenshot I don't think either of those is the case).

 I don't see any item error messages, so you should be good to finish the room, strange 🤔