To avoid a penalty for underpayment of 2021 taxes, you should make an estimated tax payment to cover the taxes on the capital gain.
You are past the first three quarterly deadlines for the estimated payments but can pay the full estimate by January 15, 2022.
Here are IRS instructions, forms, and worksheets
Your state will also have their own estimate forms.
You can pay directly on the IRS website https://www.irs.gov/payments
Be sure to mark the right kind of payment and year.....2021 Estimate.
