Missing Mail.hold in monitoring configuration

Hello everyone.I wanted to configure a notification for all administrators in case a mail is set to hold in a mailbox.

I configured an event generator unter statistics in the monitoring configuration. But i could not add Mail.hold as a statistic to monitor. It says “ERROR: Servererror: Entry not found in index”.

Do I have to create Mail.hold somewhere else? I was not successful in finding any hints towards my problem.

Thank you very much.