If the sponsoring spouse (or joint sponsor, if any) did not file taxes in the previous year, they will need to include an exemption letter explaining why no taxes were filed.
The spouse seeking a green card, either from within or outside the United States, does not need to include an exemption letter if no U.S. federal taxes were filed, and they also do not need to include any foreign tax returns.
Learn more here about what to do if either spouse did not file any taxes.