How to enable Metric Alert?

I have set the following two options, but I can’t see Metric Alerts at the alert page.

How to enable Metric Alert?

Which version of Sentry is this?

21.1.0

These are the changes needed to enable metric alerts: feat: Allow metric alerts to be used in on-prem by wedamija · Pull Request #735 · getsentry/onpremise · GitHub

Very likely you have the changes in the docker-compose.yml file already as you are using 21.1.0 so you just need to update your config file and restart your instances.