Ge 4:6 So the LORD said to Cain, "Why are you angry? And why has your countenance fallen?

7 "If you do well, will you not be accepted? And if you do not do well, sin lies at the door. And its desire is for you, but you should rule over it."

If Cain did well, that is, if he offered correctly, he would have been accepted.

Moses is teaching his Israelite readers the importance of a blood sacrifice. They will be accepted on the merit of their sacrifice, a sacrifice which looked forward ultimately to the sacrifice of Christ.