Silly question -- but are you right-clicking (or the Console equivalent) the chair to make sure its placement is flat upon the floor and/or have you tried using Surface mode?
Hello,
I am not sure if you are new to housing or not: If not...interactable chairs etc. need to be placed on a flat underground. If the underground is good, then at one point the furniture will turn green when placing it. If you place it down while green, you can interact with it. If you place it down while yellow, you can not interact with it.
Also: If you place something else on that furniture (book, ...) then it will be non-interactable again.
PS: Oh I think we are in the same guild; maybe I can show you later this evening?