The IRS uses the term "calculation error" generically to refer to many kinds of errors on tax returns, which often include software user errors.
https://finance.yahoo.com/news/got-irs-math-error-letter-152411992.html?fr=sycsrp_catchall
TurboTax has no way to know whether you received the 1st and/or 2nd EIP payments. When you prepared your tax return you were asked in the Federal Review section if you had received those payments. If you said you did not receive the payment(s) then the stimulus payment was put on line 30 of your tax return as a recovery rebate credit. The IRS cross checks every return with an amount on line 30. If the IRS sees that they already issued the EIP payment(s) to you then they adjust your refund or you even end up with tax due.
Check your IRS account---does the IRS say they sent you a payment that you did not receive? (If you filed a joint return you need to check for both spouses.) If so you can ask for a trace to be put on the payment.
https://www.irs.gov/payments/view-your-tax-account
If you were issued a payment and have not received it, see How do I request a payment trace to track my first or second Economic Impact Payments?
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
