Do not omit the salt of your God’s covenant from your offerings… (Leviticus 2:13)
This is a two-part mitzvah: an obligation to salt (in the next mitzvah) and a prohibition against neglecting to salt. Failure to perform a positive mitzvah is not necessarily a “sin” per se. If one doesn’t give charity, for example, he has merely failed to perform an obligation. The case here, however, is similar to that of Shabbos: it is a positive mitzvah to rest on Shabbos and a negative mitzvah not to perform labor on Shabbos. If one fails to observe the obligation, he consequently violates the prohibition. Here, too, one is required to salt a sacrifice. Failure to do so is more than just the lack of performing an asei, it violates a lav.
The reason for this mitzvah is that the meat would not be suitable for people without salting; does God deserve any less? Without salt, food doesn’t taste as good, smell as good, or last as long. Salt is an integral component of human culinary arts, so it would be inappropriate to omit it from the Divine.
Even though one was required to salt an offering, if it was omitted, the sacrifice was still valid, except for a flour offering, which was invalid without salt, as per our verse, which singles out the flour offerings.
This prohibition applied to male kohanim in Temple times. It is discussed in the Talmudic tractate of Menachos on pages 20a-21b and is codified in the Mishneh Torah in the fifth chapter of Hilchos Issurei HaMizbe’ach. It is #99 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos.