They look like ceiling acoustic tiles that someone has stuck to the wall. Pry them off, repair, replace drywall, or re-plaster the wall underneath, then prime and wallpaper as you like if they bother you enough to do that much work. They were probably put up to hide underlying damage to the walls.

I'm sure the asbestos police will be here to warn you of the potential dangers of such tiles, so I won't.

Nothing will stick to the acoustic tiles. You'll have to drywall over them if you want to wallpaper. I would not disturb them. You have no idea what horror they might be holding back from view.

Some of us are compelled to fix the underlying horrors in our homes rather than (literally) papering over them.

Use 3/16" plywood to cover them, or use cheap beadboard sheets, then paint.

Thank you both. We had an asbestos abatement company come out for floor tiles and those were in full view of the area they inspected and they didn’t seem concerned about the closet tiles. Sounds like I’ll need to hire a contractor. Genuinely didn’t realize I’d need to redo the walls entirely, I know nothing about home improvement (clearly). Thank you for the input!

IMO take them out but they could contain asbestos so maybe just cover them with 1/4 ; drywall n mud and tape it and then do the wallpaper which BTW will pretty much be lost in a closet

Remove them. Trying to cover them with beadboard or drywall is going to result in a cobbled up mess around the door opening.