I'm not sure, i get this on APM's app settings
Ah, I see. If you have migrated to data streams, then you should really be setting up Elastic Agent with Fleet. However...
So I assumed I couldn't use legacy apm anymore, is this correct?
In 7.17 you can configure legacy APM Server to write to data streams by setting apm-server.data_streams.enabled: true in apm-server.yml. This is the default in 8.0+