2 The LORD said to Moses: “Tell your brother Aaron that he is not to come whenever he chooses into the Most Holy Place behind the curtain in front of the atonement cover on the ark, or else he will die. For I will appear in the cloud over the atonement cover.

Read Leviticus 16:2 Using Other Translations

And the LORD said unto Moses, Speak unto Aaron thy brother, that he come not at all times into the holy place within the vail before the mercy seat, which is upon the ark; that he die not: for I will appear in the cloud upon the mercy seat.
and the LORD said to Moses, "Tell Aaron your brother not to come at any time into the Holy Place inside the veil, before the mercy seat that is on the ark, so that he may not die. For I will appear in the cloud over the mercy seat.
The LORD said to Moses, “Warn your brother, Aaron, not to enter the Most Holy Place behind the inner curtain whenever he chooses; if he does, he will die. For the Ark’s cover—the place of atonement—is there, and I myself am present in the cloud above the atonement cover.