If the letter doesn't give you an explanation, then you should call the IRS using the phone number on the letter.
One possibility -- since the amount is exactly $500 more than what TurboTax reflects for tax -- is that the IRS is assessing a repayment of the First-Time Homebuyer Credit for a home purchased in 2008. Five-hundred dollars is the most common repayment amount of this credit.
According to the TurboTax Help article How do I repay the 2008 first-time homebuyer credit?:
If you took advantage of the 2008 First-Time Homebuyer Credit (which was actually an interest-free loan), you'll repay it in equal portions for 15 years, starting with your 2010 tax return.
Your repayment is an additional tax that will either lower your refund or increase your tax bill through tax year 2025. If you got the full $7,500 credit, the additional tax is $500 per year; otherwise, if you got less than that, your repayment is 6.66% of the total loan amount.
You might also request a tax transcript to help you understand what's going on. See the IRS' Transcript Types and Ways to Order Them for guidance.
You can then use Section 8A, Master File Codes - Transaction, MF and IDRS Collection Status, Freeze and IDRS Status 48, Restrictive and Filing Requirements in the 2022 version of Document 6209 - ADP and IDRS Information to decipher what the transcript is reflecting.
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