There is a difference between פתח (entrance) and דלת (door): The space where one enters is called a פתח, while that which obstructs and locks the space is called a דלת. If a curtain is spread out there, it will say מסך פתח המשכן (screen of the door of the Tabernacle).

Thus, when it is written ושחטו פתח אהל מועד (and kill it at the door of the tent of meeting)⁵⁹⁶, it means that the entrance must be open; it does not mean נגד הפתח (against the door), since it is also written לפני אהל מועד (before the tent of meeting)⁵⁹⁷.

footnotes: ⁵⁹⁶ Leviticus 3:2. ⁵⁹⁷ Leviticus 3:8, 3:13.