Is the letter from the IRS, and does it have letter number LTR 12C at the top right? If so, the IRS is saying that you should have filed Form 8962 because you had health insurance from the Marketplace ("Obamacare"). You should have received Form 1095-A from the Marketplace. TurboTax will create Form 8962 when you enter your Form 1095-A.
The tag below your question says you are using TurboTax Online. You can no longer work on a 2023 tax return with TurboTax Online. You will have to obtain and install a copy of the desktop TurboTax software for 2023, and download the .tax2023 data file for your 2023 tax return onto the computer where you install the 2023 software. You could try calling TurboTax customer service and see if they will give you a free download of the 2023 software.
See the following for more information.
I got a 12C letter from the IRS (Form 8962)
Understanding Your Letter 12C
How do I switch from TurboTax Online to TurboTax Desktop?