As discussed under Mosiah 27:13, scribe 2 of 𝓟 tended to accidentally write the plural transgressions in place of the singular. Sometimes he immediately corrected his error (in Mosiah 27:13 and Alma 3:6 he erased the plural s). For this instance in Alma 9:23, Oliver Cowdery crossed out the plural s when he proofed 𝓟 against 𝓞.
As already discussed under Jarom 1:10, the original text prefers the singular transgression in the expression “to fall into transgression(s)”, 15 times in all. The critical text will therefore accept Oliver Cowdery’s corrected reading here in Alma 9:23.
Summary: Maintain the singular transgression in Alma 9:23, the corrected reading in 𝓟; usage elsewhere consistently supports the singular in the expression “to fall into transgression(s)”.