Here the original manuscript twice reads can. Oliver Cowdery copied the first instance of can as could in the printer’s manuscript, probably accidentally. Parallelism supports the use of the modal can in both instances.
Summary: Restore the original can found in the first sentence of Alma 36:21 (“there can be nothing so exquisite and so bitter as was my pains”), which parallels the use of can later on in the verse (“there can be nothing so exquisite and sweet as was my joy”).