Let me show you how to retrieve and complete form 8962 after your return has already been e-filed, accepted and the IRS has requested that you send them a copy. Usually the IRS requests that you send in your form 1095-A (mailed to you from the marketplace) and form 8962 (generated from TurboTax).
Assuming your return has already been accepted, in order to complete form 8962, you will have to amend your return.
Please see instructions below that explains how to complete amendment and form 8962.
https://ttlc.intuit.com/replies/5824742