I got a refund on taxes for 2020, but now find I underpaied estimated tax by $2.00. How can this be?

The only way to be certain what has changed on your tax return is to compare line by line, your 2020 and 2021 tax returns. You may find an error, or a change in income or withholding.

Print both returns and compare each page, making note of what is different. 

If you have a change in income and/ or withholding, you can review your Form W4 information and make adjustments if needed. You can review your W4 and Estimated taxes in TurboTax under the section called Other Tax Situations.

Regarding Form 2210, if your underpayment is only $2, you should opt out of completing the form and let the IRS bill you if they want to. It isn't likely they would bill you for such a small amount. 

You may have more questions once you have compared your returns, so feel free to return to Community.