that error tends to happen when you have provisioned a user, enabled them for archiving and then they are removed from the provisioning group
this can happen when you delete a mailbox from active directory etc without disabling them first
this error should not stop you from enabling other users in the provisioning group
you should be able to see the provisioning report to determine what user it says is enabled but is not part of the provisioning group
https://www.linkedin.com/in/alex-allen-turl-07370146