Where was Aaron warned not to enter freely, or else he would die?

Answer: The Most Holy Place

Aaron was not to enter the Most Holy Place behind the veil freely, for God appeared in the cloud above the mercy seat.

“And the LORD said to Moses: “Tell your brother Aaron not to enter freely into the Most Holy Place behind the veil in front of the mercy seat on the ark, or else he will die, because I appear in the cloud above the mercy seat.”— Leviticus 16:2 (BSB)