The original manuscript reads great loss without a preceding indefinite article a. But when Oliver Cowdery copied this phrase into the printer’s manuscript, he started to write “a great loss”; he actually wrote only the a, but then he immediately overwrote it with the initial g of great and continued inline with the rest of the word, reat.
Elsewhere in the text, there are two occurrences of great loss without the indefinite article a:
In the first example, we can see Oliver Cowdery’s tendency to insert the indefinite article before great loss: Oliver initially wrote a greater loss in 𝓞, but then he immediately corrected the noun phrase to great loss by erasing the indefinite article a and the -er ending. The second example, in Alma 63:15, provides specific support for great loss in Alma 57:23 since both passages refer to “suffering great loss”. Thus the use of great loss without the indefinite article is supported by usage elsewhere in the Book of Mormon text.
It should be noted that there are three instances of great loss where there is a determiner but never the indefinite article a:
Summary: Accept Oliver Cowdery’s deletion of the indefinite article a before great loss in Alma 57:23, the reading in 𝓞 and the corrected reading in 𝓟.