If you received a Form 1095-A for your health insurance through one of the state Marketplace Exchanges or from healthcare.gov and entered the 1095-A on the 2023 tax return, then the TurboTax software would have completed the Form 8962, Premium Tax Credit.
Since the IRS sent you a 12C letter you did not enter the Form 1095-A on the 2023 tax return.
See this TurboTax support FAQ for the 12C letter - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-forms/got-12c-letter-irs-form-8962/L...
