You can't search for admin audit log entries because the arbitration mailbox for the organization is located on a server that doesn't have Exchange 2013 installed.
Issue: You can't search for admin audit log entries because
the arbitration mailbox
'SystemMailbox{e0dc1c29-89c3-4034-b678-e6c29d823ed9}' for
the organization '' is located on a server that doesn't have Exchange 2013 installed. The mailbox must be moved to an
Exchange 2013 server before you can search for admin audit log entries.
Resolution: Move arbitration mailbox to Exchange 2013
Get-Mailbox -Arbitration -Identity
"SystemMailbox{e0dc1c29-89c3-4034-b678-e6c29d823ed9}" |
New-MoveRequest -TargetDatabase <name of Exchange 2013 database>