en_tn/lev/16/17.md

he must go out to the altar that is before Yahweh

This is the altar of sacrifice just inside the courtyard of the tabernacle.

and make atonement for it

Like the holy place and tent of meeting, the altar is unclean because of the sins of the people.

horns of the altar

This refers to the corners of the altar. See how you translated this in Leviticus 4:6.