Without additional context we aren't going to be much help. But just reading the error, the code that is running for whatever event just occurred (e.g. document saved) is attempting to execute a command in Word. However Word does not allow that command while it is handling the event and fails the call. For example, if the code was called when the document was saved and the code attempted to save the document then I could see this error potentially occurring (because you're already trying to save).
Again, without any additional information including the code that is running and what "event" it is talking about we are just guessing here.