It was Cain's action of keeping a portion of the offering for himself. Not the offering itself.
Hebrews 9:26You don't learn that until Hebrews 9:26.
26 For then must he often have suffered since the foundation of the world: but now once in the bend of the world hath he appeared to put away sin by the sacrifice of himself.
What does this verse have to do with Cain and his offering?