If no one from your instance has subscribed to the community, it can not be found normally via search and the content wont be federated to your instance.
To be the first to connect your instance to that specific community: type the real url (https://kbin.social/m/functionalprint) into the search with search setting set to "All" and you should find that community. Then just subscribe and wait for your instance to start federating with that community and you should be able to participate.
Once you have done this others in your instance should see the community listed and can instantly jump in.
Edit: I've explaned this here with a picture.