Mitzvah #121 is the obligation to bring a korban chatas (sin offering) in certain cases; Mitzvah #138 is the obligation to follow the specific procedures of a korban chatas. Similarly, Mitzvah #129 is the obligation to bring a korban asham (guilt offering) under certain circumstances, while here we have a command to follow the procedures that apply to such a sacrifice.
Unlike the chatas, which could be brought from a wide variety of species, the asham was limited to male sheep, though from a variety of ages depending on the sacrifice.
As we said in Mitzvah #138, the Sefer HaChinuch says that we cannot discern the meaning of every detail of every sacrifice. The “big picture” reason for sacrifices is to draw us closer to God. The individual details facilitate this in ways we cannot understand.
The Talmud in Menachos (110a) tells us that the phrases “zos toras hachatas” (“this is the law of the sin offering”) and “zos toras ha’asham” (“this is the law of the guilt offering”) teach us that one who studies the Torah on these sections is credited as if he had actually offered the sacrifices themselves!
This mitzvah applies to male kohanim in Temple times. It is discussed in the Talmud in tractate Zevachim, on pages 48-50, and in Kerisos on 22b. It is codified in the Mishneh Torah in the ninth chapter of Hilchos Maaseh HaKarbanos. It is #65 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos. As mentioned, the Ramban (Nachmanides) does not include the obligations of the various sacrifices in his count of the 613 mitzvos.