I thought that if I entered a Fediverse address here, like '@FrankM@nrw.social,' it would follow that user, and their posts would then appear in the corresponding category.
But that's not happening. There's also no meaningful error message. The actor you see in the image probably appears because I entered an incorrect address.
Does this feature need to be enabled on the Fediverse instance 'nrw.social'?
Who can help?
nrw.social is a mastodon instance, which does not create group actors. Lemmy, Mbin and PieFed do. In Lemmy terms, a group actor is called a "Community". Every lemmy instance lists their communities at /communities, e.g. https://lemmy.world/communities.
Try putting "https://lemmy.world/c/technology" into that field and see what happens.