You can not use Exchange Management Console (EMC) to grant Receive As permission for a mailbox database.
You must have a user who has Receive As permissions for the Exchange mailstore and is not explicitly denied this right due to group membership.
Use the following cmdlet:
Add-ADPermission -Identity "MDB01" -User "ServiceAccount" -ExtendedRights Receive-As
Above cmdlet grants the user ServiceAccount Receive As permission for mailboxes on mailbox database MDB01.
