Cannot create alert rule based on loki/cortex

I have setup grafana and prometheus on kubernetes (aks 3 node cluster) by using this guide:

So it is a setup of prometheus operator based on:

Now I have Grafana 8.4.3 running and I can see in the “alerting” section of the dashboard the “cortex / loki” rules. I’d like to setup alerting based on some of the rules. So I choose “New alert rule” but there are no datasources available when I choose “cortex/loki managed alert”.

In the datasources I have setup the prometheus data source which seems to be working.

Why are there no datasources shown when I try to create a new alert rule?

greetings roman