What did Laban say was not the custom in his land?

Answer: Marrying off the younger first

When Jacob found Leah, Laban said, "It is not our custom here to give the younger daughter in marriage before the older."

“When morning came, there was Leah! “What have you done to me?” Jacob said to Laban. “Wasn’t it for Rachel that I served you? Why have you deceived me?” Laban replied, “It is not our custom here to give the younger daughter in marriage before the older.”— Genesis 29:25-26 (BSB)