I have an idea for a game...

I am working on this simple side-scroller concept with hand drawn animations. my plan is to beta the idea on Scratch(not working for me) carrying it to unity and putting it on itch. should i jump to the unity prototype early?

 

Yes.

If you started in Scratch, none of the code will really carry over to Unity. Logic might, but you might as well just start in Unity. If you are considering Scratch, it sounds like you will still need some time to familiarize yourself with C# scripting. It's better to start that now rather than in Scratch. That's my opinion, at least. There are plenty of great resources in the Learn section.