Error:

Active Directory Operation Failed on DC1.vanarts.com. This error is not retriable. Additional information: Insufficient Access Rights to perform the operation.

Active Directory response:00002098: SecErr: DSID-03150BB9, Problem 4003 (INSUFF_ACCESS_RIGHTS), data 0

The User has insufficient access rights.

Enable-mailbox -Identity 'vanarts.com/dennis' -Alias 'Dennis'

Solution:

Active Directory Users and Computers