I used to get this when I worked and people still had document open, but Toes probably right, found this try it :
Disable Read-Only Mode
This problem can also occur if the document has been opened in read-only mode. In this scenario, disabling the read-only mode should fix this problem.
To disable the read-only mode, follow the steps given below:
Locate the Word document you want to edit and right-click on it.
Click on the Properties option from the pop-up menu.
Uncheck the Read-only attribute in the General tab.
Now, select the Security tab and see if the elements in the Group or user names section have been allowed the Full control, Modify, Read & execute, Read, Write permissions.
If they are not allowed then, click on the Edit option below the Group or user names section and select the boxes for the options given above.
Finally, click on Apply and OK to save all the changes.
Now, check if you still can’t edit Word document. This solution should fix this problem.