Here in the printer’s manuscript for Omni 1:17, Oliver Cowdery supralinearly inserted the conjunction and (written as an ampersand), probably later as he was proofing 𝓟 against 𝓞 (the inserted & is in heavier ink flow). See the previous discussion regarding the inserted many in this verse. The and here is expected.
In his editing for the 1837 edition, Joseph Smith deleted the not that originally occurred in this verse (“and Mosiah nor the people of Mosiah could not understand them”). As discussed under 2 Nephi 23:17, the earliest text had three occurrences of the conjunctive nor within a subject noun phrase followed by a not in the associated verb phrase:
As shown by the use of dashes, the nor-phrase acts parenthetically. The critical text will maintain the original not in all three of these cases, including here in Omni 1:17.
Summary: Restore the original reading in Omni 1:17 (the corrected reading in the printer’s manuscript): “and Mosiah—nor the people of Mosiah—could not understand them”; this construction is found two other times in the original text.