Written explanation why i did not file one of the three required tax returns to become I864 joint sponsor.

A simple written statement or letter signed by you and notarized and explaining why you did not pay taxes for the year in question, (just for example: you were unemployed or were a student or were not present in the country). It would be wise to discuss in more detail with, or have your letter reviewed by, an experienced immigration attorney to make sure that your letter does not raise any issues that would interfere with your eligibility to be a joint sponsor.