It doesn't matter whether you used the insurance or not. What matters is that you had the insurance. There's no way to undo that now. You should have gotten a Form 1095-A from the Marketplace. You have to enter it in your tax return. When you enter the 1095-A TurboTax will produce Form 8962.
It sounds like you got a letter from the IRS (LTR 12C) asking for Form 8962. Follow the instructions in the FAQ at the following link. At the bottom of the FAQ click the link for the type of TurboTax software that you are using.
I got a 12C letter from the IRS (Form 8962)
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