Let's break this down, because you might be talking about two different situations.
1. UEC is always taxable income, it is reported on a 1099-G from the state and you must enter it on your tax return. Any tax withheld is only an estimate. Your actual tax is calculated on your tax return, and might be more or less than the withholding, so you might owe more tax or be due a refund. If you did not report your UEC on your tax return, you should prepare an amended return to calculate the correct tax, and send that amount along with a copy of the amended return to the IRS office that issued the notice. You probably also need to amend your state tax return.
2. The IRS might be looking at your Cashapp records and thinking that the money deposited on the cash app came from outside work instead of your own funds, so they think you are running a side business under the table. In that case, you don't owe tax, but you do need to respond to the IRS notice by sending a letter explaining your situation. Include a spreadsheet, transaction records, receipts, or other documents showing that the funds on Cashapp came from your UEC and not from an outside job.
