The original text here uses the subjunctive form know rather than the indicative knows (or knoweth). In his editing for the 1837 edition, Joseph Smith added the s to know.
Elsewhere the text has examples of both the subjunctive and the indicative in subordinate clauses headed by save. There is one clear example with the pronoun he that takes a subjunctive form of the verb be:
It is possible, of course, that in writing down the text for 2 Nephi 9:20, Oliver Cowdery accidentally dropped the s for knows. We see an example of this kind of error later on in the small plates:
The original manuscript is not extant here, but most probably it read “save it comes”. In the printer’s manuscript, Oliver Cowdery corrected his initial come to comes by immediately inserting the s after writing come (there is no change in the level of ink flow).
Nonetheless, there are a few examples with subjunctive forms for verbs other than be, although for the following instances the subordinate conjunction here is if rather than save:
Thus the subjunctive form know in 2 Nephi 9:16 appears to be possible, which means that the critical text will restore know to the text even though it may be an error. For additional discussion and examples, see mood in volume 3.
Summary: Restore the original subjunctive form know in 2 Nephi 9:20 (“and there is not any thing save he know it”).