When you scan an Exchange On-Premises organization you see the following error

Error when 'Reading add-ins'. Error executing the command 'Get-App'. The specified mailbox "mailbox name" doesn't exist. Reason: mailbox name isn't a mailbox user.


The account performing the scan must be associated with an Exchange mailbox in order to execute the Get-App cmdlet.


Ensure that the service account (or custom credentials if in use), has an associated Exchange mailbox.

More Information

The error can be replicated within the Exchange Admin Center.