A backup cannot be run when other users are logged into the company. Check Log out users prior to running backup if you want automatic backup to log out any users who are logged in so the backup can be run.
Whether you select this option or not, you should make sure all users are aware of when the backup is scheduled to run so they will not be logged into the company at that time. Ideally you should schedule the backup at a time when no users should be accessing Sage 50 such as 1 a.m.