Do you mean the message "Can't perform the action while another item is being used"?
It means you started using a tool/option but have not finished that task yet, iirc. Since it could be pretty much any of the task the you started and left in the middle, I can't really help you figure out which one it might be, as there are too many of them.
You could just back track to verify in case you left anything incomplete. In the worst case scenario, I guess you'll have to restart the game.