The user entered in User Details must have security access to back up the company. This user's security access under User Security>Company>File>Back up should be set to Full Access.
If you're going to set up Automatic Backup to log out users so the back up can be run, the user entered under User Details must also have security access to log out users. This user's security access under User Security>Company>Maintenance>Log Out Users should be set to Full Access.
Click Verify User Details to verify the user you have entered has the appropriate security access. User Security is maintained under Maintain>Users>Set up Security. Please see your Administrator if you have questions about a user's security access.