I am a beginer for Adobe stock. I want to know why my illustration (PNG) is rejected?

As @Jill_C pointed out, you need to improve your cutout work. To check my cutout work, I use the "stroke" effect in Photoshop. That effect shows you even the thiniest error on your asset, and that during your working on the cutout. 

BTW: it looks to me that you used an automated background remover. They are not perfect. You should do that manually.