So I've built Lakeview Manor and I'm in the process of decorating it but I can't add an arcane enchanter to the main hall. It says I don't have a grand soul gem, even though I do.

it needs to be a full one if im not mistaken.

Also tried full one.

Strange indeed...that must be a new bug.

Try to sell your full gran soul gem, make a save in a new save slot. Exit your game. Restart your console.

Load your last save, and buy a grand soul gem from another vendor in another city.

Try to get back to your home and try again. (btw. leave your follower outside from your mansion, with the wait command.)

Actually, different soul gem seems to have done the trick. Problem solved, thank you.