The original text here in Mosiah 21:21 (“carried off of their grain”) is correct, despite its difficulty for modern English readers. The of here provides a partitive sense—namely, the priests of king Noah carried off some of the grain, not all of it. The 1837 edition removed the preposition of here, perhaps intentionally. But it was not deleted by Joseph Smith when he marked up the printer’s manuscript in his editing for the 1837 edition. Interestingly, the 1908 RLDS edition did not restore the original of here, despite the fact that Joseph left the of unchanged in 𝓟. Perhaps the 1908 editors thought that off of was some kind of dittography.
A similar example of the partitive of is found earlier in the book of Mosiah; once more, the 1837 edition removed the difficult of—in this instance, by replacing it with off:
There are quite a few examples of the partitive of in the Book of Mormon text, as in Mosiah 2:3: “they also took of the firstlings of their flocks” (that is, not all of their firstlings, only some of them). For a complete list of the partitive usage “to take of X”, see under Mosiah 9:14.
Summary: Restore the original partitive of in Mosiah 21:21 (“and carried off of their grain”).