Am I allowed to submit a speedrun for someone else? Not submit someone elses run and claim it as my own but submit a run for someone else

The submission of runs not your own runs will need a real permission (by Twitch/Twitter/Discord contact) from the runner/runners so as to submit here in speedrun dot com. The runner could not have a speedrun dot com profile here, but you can submit it anyway if you do know the name of the user (alias/nickname), of course... all this must be autorized by the runner in question.

A little correction - you can submit other people runs in a game (much preferred with the runner's permission, as manticor said), but only if you are the moderator of that game. Otherwise, it will be a submission of your runs only.

Or, as I did, you can ask the mod of the game and they probably will do it.

i was wondering the same thing my brother wanted to know.