If you are trying to add an individual and see this warning, that mean that the ID number belongs to another individual (active or inactive).
If you cannot find them on the active student roster, make sure you check the inactive list. If it's the same student, you can reactivate them.
Another option is to use the ID lookup feature at https://app.5starstudents.com/Help/IdLookup. The result will include the school year(s) that the person is attached to.
If the ID belongs to a different person, you'll need to permanently remove the individual from the associated school year(s) in order to add the new student.