Did you pay more taxes during 2021 (via withholding and estimated tax payments) than you owed in 2020? If not, they can charge a penalty for underpayment. I usually do a back of the envelope calculation in the first week of January to see if my withholding and est. tax payments exceed the total tax I owed the previous year. If not, I make an extra payment in early Jan to cover it by at least $1.
