That wasn't my issue.  The 2021 estimated tax vouchers were correctly calculated as of the time the original tax return was filed.  Those vouchers did not take into account any deferral and the client correctly made these payments throughout the year.  It was when I reopened the 2020 file later that I noticed a discrepancy between what the client paid and what the software must have recalculated for the vouchers.  The software incorrectly revised the vouchers to incorporate a non-existent deferral.