Well, most instances I’ve seen so far have been generic. The most specific one I’ve seen so far was https://startrek.website.

Most instances allow people to create communities. Looks like you are on mander.xyz. I don’t know much about that one but if you don’t see a “create community” button in the sidebar that means they have it set so only moderators can create communities. You’ll have to ask a mod to create one for you, or signup on another instance like Lemmy.world and create your own community.

Basically join an instance and try to create the sub on there. if they don't let you or if there are no admins to help. create an account on another instance and if that one works then your good. people can always find you from another instance and can comment and post on whatever instance you are on. like me I created account here on lemmy.world i was on lemmy.ml at first but i couldn't create the sub. so now i have the sub and anyone can find me.