Can I create a Good sonic fan game with no coding experience and learning Fusion 2.5?

Welcome to Fusion Making Sonic-style games is definitely possible with Fusion - see all the fangames and Freedom Planet, and so on - but it's a deceptively complex game to write, with a lot of difficult physics. If there are specific tutorials on the Sonic Worlds Delta Engine, maybe you could go through those and see where you get stuck?

Otherwise, maybe step back a bit and look at some of the example Fusion tutorials just to get a feel for it and how events work and so on before moving on to bigger projects.

To create a decent fangame using the presets provided by the worlds engine? Pretty good odds if you apply yourself.

To understand and modify said engine? Probably not. Sonic the Hedgehog has always had a comparatively complicated, high-level engine. You would need to be working with games for quite a while and read several articles specifically about how Sonic works to have a good handle on what you would need to tinker with the physics directly.

If you want some insight into how the sonic game engines work, you can start here: Please login to see this link.

Also a site that has some good knowledge about games regardless is here: Please login to see this link.

If you are very determined, you can do it. I would recommend you follow DavidN's advice and choose several easier projects to familiarize yourself with Fusion and game creation in general, though. It will save you a lot of gnashing of teeth and headaches.