Hi there! Looks like you linked to a Lemmy community using a URL instead of its name, which doesn't work well for people on different instances. Try fixing it like this: !harrypotter@literature.cafe
You should just need to write !harrypotter@literature.cafe in the comment box, and the link is generated automatically. Should work in both Lemmy and kbin at least, not sure about mbin but would make sense that it works there as well. :)
Kbin needs to be aware of the community first, so my first link didn't work properly because of that. After interacting with the community once the !community@example.com format works just fine. :)
I subscribed from Kbin.social now, so future posts should be visible from there. :)