Someone Signed On With Same User ID
A person at a different workstation has already opened this company file with the user ID that you entered in the User ID field of the Sign-on window. A user ID can be used by only one person at a time.
Either enter a different user ID in the User ID field of the Sign-on window or wait until the person using the user ID you wish to use closes the company file.
If nobody else is currently using the user ID you wish to use, it's possible that the company file was closed incorrectly the last time this user ID was used. If this is the case, all current users of the company file must close the company file and then reopen the file to remove lockfiles. You should then be able to open the company file with this user ID again.
This support article has further information on resolving this issue if no one is actually signed into the file.