I've just recently joined after getting interested in speed running and a game I want to speedrun an ending to isn't available. Is there anyway I can create a speedrun category for the game?

No. Ask the mods but in most cases you won't get the category added.

thank you so much

who are the mods?

Moderators. People who control the leaderboards. People who verify runs. Manage game forums. Etc.

Did you mean you want to add a category to a leaderboard that already exists, or a leaderboard for a game that doesn't currently have one? Because you can request a game if it isn't on here.