The 1840 edition omitted the modal verb must. This omission could be a typo. On the other hand, it could represent editing since in all other instances when a Book of Mormon writer refers to ending his account, the word must is never used. There are 27 examples of “had made an end”, 15 of “make an end”, and one of “having made an end”. For instance, at the beginning of the next chapter, Nephi writes: “and now I Nephi make an end of my prophesying unto you” (2 Nephi 31:1), with no use of must.
The RLDS textual tradition followed the 1840 reading until the 1908 edition, where the must was restored, in accord with its occurrence in the printer’s manuscript. The 1981 LDS edition removed the must, undoubtedly by reference to the 1840 edition.
We do not have the original manuscript here, so we cannot tell if it read must or not. Nonetheless, the accidental insertion of must seems to be a highly unlikely error in the transmission of the Book of Mormon text. There are no examples elsewhere in the entire textual history (in either the manuscripts or the printed editions) where the modal verb must has been either deleted or added. (On the other hand, there are numerous examples involving the deletion and addition of the future modal verbs will and shall.) Although the use of must is unique in this context of “making an end” of speaking or writing, it seems perfectly reasonable: Nephi here feels moved upon or perhaps even obligated to end his sayings.
Summary: Restore must in 2 Nephi 30:18 (“I must make an end of my sayings”) since it seems to be intentionally used here.