The original manuscript has the singular transgression, which Oliver Cowdery accidentally copied into the printer’s manuscript as the plural transgressions. Elsewhere in the Book of Mormon text, every occurrence of “because of transgression(s)” is in the singular, never the plural:
In one example, Alma 3:6, scribe 2 of 𝓟 initially wrote transgressions, but then he immediately erased the plural s (thus showing once more the tendency in this context to write the plural transgressions instead of the correct singular).
Summary: Restore in Alma 37:42 the singular transgression, the reading in 𝓞 (“because of their transgression”).