Rename Label shows no Options for Fields returned by query

Hey Guys,

we are using Grafana to show Health data from different Servers. The Sources are Grafana (via Plesk) and everything works like a charme.

I would like to merge same data from different Servers in one really simple panel

Now all labels are named the same, because the metric is the same from different servers.
I would like to rename the legends and found the right way by “Fields returned by query”.

But there is no result:
Screenshot 2024-09-06 092015

What am i missing?