The 1840 edition replaced the determiner this with the definite article the before the noun scripture. This change may be due to Joseph Smith’s editing for that edition, or it may simply represent a typo on the part of the 1840 typesetter. Either reading is possible, which suggests that the 1840 change is simply a typo. Elsewhere, it is true that the text has only the scripture, never this scripture:
David Calabro also points out (personal communication) that here in Alma 12:21 an original the could have been misheard as this as Joseph Smith dictated the text to Oliver Cowdery (the scribe for this part of 𝓞), especially since scripture, the following word, begins with the letter s. For another example involving this versus the, see the discussion under 1 Nephi 15:21; there the original text apparently read “what meaneth the things which our father saw in a dream” (rather than the current reading, “what meaneth this thing which our father saw in a dream”).
Despite this internal evidence for the scripture in Alma 12:21, there are other examples involving the verb mean that occur with either this or these and are followed by a relative clause headed by which:
Thus the use of this in Alma 12:21 is perfectly acceptable and will be restored in the critical text.
Summary: Restore in Alma 12:21 the demonstrative this, the reading of the earliest textual sources; the reading “what does this scripture mean” is supported by the nearby usage in Alma 12:8: “what does this mean which Amulek hath spoken”; the replacement of this by the in the 1840 edition is probably a typo rather than the result of conscious editing.