so i am halfway dont with a platformer game. Am i able to use this for the Jam or are all games supposed to be completely built during the Jam window?

Has to be a fresh project started after the start date. Doesn't mean you can't practice before hand so it's second nature when you start fresh. :D

Highly recommend you also polish the first 5 - 10minutes of gameplay. Last year that's all the time they could spare