You have a query, report or form open that uses at least that table?
Or you have a version of that db open that perhaps didn't terminate properly, in which case, check Task Manager for more than one open instance.
You have run code that has put one or both of the tables in edit mode.
Since it's not possible that it is a shared db, then doubtful someone else has a lock on that table, otherwise also a reason why you'd get that message.
Those would be my shots in the dark.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.