God said, “Why are you so angry, Cain. You know the requirement for sin. I’ll give you a second chance to make it right.”
Instead, Cain confronted Abel, out in the field, and killed his own flesh and blood. Later, God heard the blood of Abel calling out to Him from the field. Abel has been in Heaven for 6,000 yrs.; Cain’s been in Sheol.
Many people have accepted God’s way of salvation through His Son Jesus’ suffering and death on the cross. They know every drop of His blood was emptied from His body. Young Abel may not have understood God’s requirement for sin, but he obeyed and found favor with God.
Many people are still trying to please God in their own way like Cain did and God can’t respect it. God loves and appreciates our good works for people, but they aren’t good enough to cover our sins and get us into Heaven. Something valuable of a great price like God’s righteous Son’s good work on the cross is required to paid for our sins and He is worthy to complete our salvation.