Locked by another user
Current versions of Word let you edit collaboratively with another user. However there are times when you'll get a "locked by another user" error. There are several causes, such as someone (including you) checking out a document on SharePoint rather than just opening it, using an earlier version of Word, or having a crash while you have a document open.
If the other user is you:
Close the documents on the other device where you have it open.
You can edit a copy and then "Save as" and overwrite the original file.
If the document is locked due to an earlier crash, try rebooting your computer to clear it.
If the other user is really someone else:
Contact the other user and ask them to check the document in. For more info, see Check out or check in files in a document library.
Ask the user to open the document on a version of Word where you can collaborate. The document may need to be shared.
Either you or the other user edit a copy and reconcile differences later. For more info, see Combine document revisions.
To avoid these issues later, see Collaborate in Word.