Question: If eating animals was not allowed at the time of Abel, how could he make an animal sacrifice?
Answer: Animal sacrifice isn’t similar to eating animals. Animal sacrifice is a way of coming close to G-d (sacrifice in Hebrew is called korban, from the word korev which means to come close). The offerer should take a live animal and see himself in the animal. He should think that it should be himself being offered, but G-d wants him to live. He should see the animals blood instead of his, the animals limbs instead of his, etc. This isn’t only an elevation for himself, but also for the animal since the animal is now elevated instead of mundane.
All the Best,
Rabbi Meir Goldberg