Some schools/centers do not participate in the PennO365 email tenant. If the currently-listed school/center does not qualify for PennO365 email, it will say so in the Enrollment Tool in column "O365 ready".
Mailbox Enable the account.
Wait 30 minutes after the override and mailbox enable the account in ARS.
Confirm they have an account in the Enrollment Tool under the column "Has mailbox."