The 1830 compositor set “preparation for war”; the printer’s manuscript has the plural preparations (spelled as preperations). The 1837 edition restored the plural preparations. The word is partially extant in the original manuscript, but not the end of the word.
This plural reading is consistent with the use of the plural preparations in the next two verses:
More generally in the text, when the verb is make, there are 17 examples where the direct object is the plural preparations (including here in Alma 24:4), but there are none that have the singular preparation, although occasionally the plural has been replaced by the singular (see under Jarom 1:8, Mormon 2:4, and Mormon 4:6). The plural use in “to make preparations” is quite correct in Alma 24:4.
Summary: Retain the plural preparations in Alma 24:4, a reading that is consistent with 16 other examples of “to make preparations” in the text.