I had an issue where I was moving all users from one database to another.
The plan was to delete the original database but I wasn't able to, this was because I had to move the arbitration mailbox.
Open up Exchange Powershell and type in:
Get-Mailbox -Arbitration | New-MoveRequest -TargetDatabase "name of new database"
After running the above command , Arbitration will be moved and you'll be able to delete the old database.