You shouldn't use a Game Jam to learn a new engine. After all, writing a game engine is something that can take months or years, and if all you have is a weekend, you want to spend your time working on the actual gameplay and not fiddling with low-level initializations.

Virtually all Game Jams allow you to start with a working "base code" of routines that implement common game development tasks. You are rarely forced to truly start from scratch, so most Game Jammers use a pre-existing game engine and start from there.

If you already know how to program games, you likely already have a favorite game engine. It could be one you've made yourself, or an engine that you know and love from previous projects.