After my return was "accepted" through turbo, the irs sent me a letter saying I need to file an additional form. Can I file through turbotax or do i need to mail it in?

Was it Form 8962?

What form would that be?

Yes it was 8962

You should do exactly what the IRS is requesting so that your return will be processed as quickly as possible. You'll need to mail it in.

You'll actually start an amended return in TurboTax to create the Form 8962 that the IRS needs.  Please see the TurboTax FAQ "The IRS is requesting Form 8962 for 2018" for additional guidance.  Note that there are different ways to do this, based on whatever version of TurboTax you're using.  They're shown in the three dropdowns near the bottom of the FAQ.

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