You cannot delete a user group that is not empty unless you specify the -force parameter. If you use the -force parameter when you delete a user group, all of the users that were in the deleted user group are added to the Monitor user group.
Ensure that you specify the correct user group. For each member of the specified user group that you want to belong to a user group other than Monitor, move that member to the desired group. If the user group has at least one member, specify the -force parameter when you submit the task.