Error assigning keybox to a listing

If you are trying to assign a keybox to a listing in Matrix and get a message that the keybox is already assigned to another listing, you must first unassign it from the listing it is currently assigned to.  If you do not unassign it first, you will get an error similar to the following:

Simply remove the keybox from its current assignment and then you should be able to assign it to the new listing.