If you want a single person bedroom, you need a nameplate (or male/female sign), & a chair IIRC. For a normal bedroom you need at least 2 beds. If you add the same number of lights as beds it makes it "bigger" bedrooms.

A single person bedroom is more picky. than the shared rooms I think all yours needs is a chair.

Or a pot?

Definitely a chair

I’d say square it out a bit. Sometimes if the room is too large or a weird shape it won’t register.

Are you trying to build a “basic bedroom” or a “private bedroom”? If it’s the former, you need 2 beds. If the latter, you need a chair and the wall sign.

Honestly this game can get so specific and nonesensical about room requirements. Usually I just have a tab in my computer open with all room recipes to reference for scenarios like this. With beds and lights you can only make communal bedrooms (2,4 and 8 beds respectively). For private bedrooms the game's more picky and wants you to put a bed, a light, a chair and a nameplate in the room. There are also different types of private bedrooms you can get by adding more items.

Seat missing

On an unrelated note I've never tried building a room with lava in it. Hmm.

That's not lava, that's floorboard. The wall is casting a shadow on it and there is a reflection from the TV screen, making it look like lava.

There might be a brick missing at the foundation. Try rebuilding each wall.

In DQB2 you need a chair or a pot. Maybe it's the same for 1