I would try it in the Settings, there’s a Background section. Ah, I see it’s also based on the Gallery.
If the stock Gallery doesn’t work for any reason (it would not be a wonder) you could install another, second gallery app like Aves. This one will work.
I’ve found that long press the background (in Lawnchair) and wallpaper does not allow to set a wallpaper, however settings/wallpaper & style does allow this. That is quite confusing.