When installing exchange mailbox role you get error : “Database is mandatory on UserMailbox”

This can happen if you have users type mailbox without belonging to mailbox database

To find those users, execute powershell command :

Get-Mailbox -arbitration | Select Name, Database

This will show you the problematic users. You can fix this by add the users to database executing for each user :

Set-Mailbox "user name from the command above" -Arbiration -Database "Mailbox database name of your choice"

This will let you continue the exchange installation to the end

Good Luck