Hi i just wanted to ask if i could speedrun just one level out of any game.
Well for every game that has levels you can just speedrun one level but not all games with levels have a level leaderboard on speedrun.com. Check the leaderboard for the game you're thinking of, if it has a "Level Leaderboard" on the left side then you can submit your speedrun. If it isn't there you can still speedrun the level, you just won't be able to submit it to the site.
Speedrunning single levels are called "IL runs" and yes, you can speedrun a single level if you want. As @TRLittleToaster said, check the sidebar to see if a game has a level leaderboard. If it doesn't you can still speedrun whatever you want, but you won't be able to submit it.
"Can I speedrun...."
Yes.
That's the only answer needed for that question. Speedrun anything you want.
If you want to know whether the communities for the games in question will set leaderboards for what you want to run, you should ask those communities directly.
You can do Indiviudal Level speedruns for any kind of game, but for most games IL speedruns are just for practice attempts of longer categories
Only a few genres are dedicated to IL runs being a main category over ANY% - typically Racing Games, Real Time Strategy Games, and games that forcibly return you to a stage select main menu with no overworld whatsoever.
There are a couple of comunties in genres that would normally be ANY% who have chosen to favor ILs as well - Goldenye 007, Super Monkey Ball, etc.
I personally prefer individual levels because they allow for really extreme strategies, while most Full Game type speedruns will usually being their life with a lot of safety strategies and then over years gradually get pared down (causing a lot of mental stress) as players play more dangerously.
