Why did Saul first tell David he could not go out and fight Goliath?

Answer: He was just a youth

Saul said, "You are just a boy, and he has been a warrior from his youth" (KJV: "thou art but a youth").

“But Saul replied, “You cannot go out against this Philistine to fight him. You are just a boy, and he has been a warrior from his youth.””— 1 Samuel 17:33 (BSB)