The two most common causes of the "selection is locked" message are (1) an unactivated version of Office [apparently not your issue since the problem is with only the one file] and (2) a document that is protected in some way, usually for forms using legacy form fields.
You can try going to Tools> Protect Document but if a password has been applied it will do no good. Also, if the protection was applied in a Word for Windows PC edition of the program you may not be able to remove the protection on a Mac... PC Word has much more robust internal document protection components.