Who sent you the letter?
If you received unemployment compensation in 2021 and did not receive a Form 1099-G from your state you were still required to report the unemployment compensation you received on your tax returns.
perhaps someone impersonated you and got those UC benefits. The place to start is with the UC bureau to make sure it was really you that got the benefits. if it was not then send back the letter to the IRS with a letter of explanation that you were a victim of identity theft and include form 14039.
https://www.irs.gov/pub/irs-pdf/f14039.pdf
if it was you pay up. you owe the taxes and the IRS probably won't abate penalties unless you can provide a reasonable cause. Not getting the 1099-G is not reasonable cause because you should have been aware that you got the money.
Did you receive unemployment benefits in 2021? If so, as others have said, it's your responsibility to report the income. The state would have issued a Form 1099-G, but some states don't send the 1099-G form in the mail. You have to access it on the state's unemployment web site. Check with your state unemployment office.
Not getting a document (
".....unemployment that I never got a tax document to claim....")
does not mean you were not required to enter the unemployment compensation on your tax return. Many states expect you to get it yourself from their unemployment website.
You can amend your return to include the 1099G:
See this TurboTax support FAQ for amending a tax return - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-change-correct-return-a...
Or....if you amend and find out that you owe the amount the IRS has already told you to pay, then just "pay up" as another Champ has already suggested.
Unemployment compensation is taxable on your federal return.
You will have to enter a 1099G that is issued by your state.
Some states will mail out the 1099G. Or you might need to go to your state’s unemployment website and use the password, etc. that you have been using to certify for weekly benefits to get your 1099G from the state’s site.
Enter your 1099G in Federal>Wages & Income>Unemployment
Go through the screens very carefully, making sure to enter any federal/state tax you had withheld from the unemployment.