As explained under Alma 24:4, the original text of the Book of Mormon consistently uses the plural preparations in the phrase “to make preparation(s)”, but the tendency in the history of the text has been to replace the plural with the singular preparation. Here in Mormon 2:4, the 1840 edition made this change; the RLDS text restored the correct plural in the 1908 RLDS edition. See nearby under Mormon 4:6 for another case where this change to the singular occurred.
Summary: Maintain the plural preparations in Mormon 2:4, the reading of the earliest textual sources; the Book of Mormon text always uses the plural preparations in the phrase “to make preparation(s)”.