I filed my 2020 taxes already and my federal was accepted. I received a form 1095-A after I had already submitted. Was that something I needed to add ? How can I fix it?

Yes, you need to include the 1095-A information if you received the Advanced Premium Tax Credit.  You will want to amend your tax return when available to include the information, but amended tax returns.  See How do I amend my 2020 TurboTax Online return?

If you receive a Form 1095-A, Health Insurance Marketplace Statement, showing that advance payments of the premium tax credit were paid for coverage for you or your family member, you must include it on your individual income tax return and submit a Form 8962 to reconcile those advance payments, even if you would not otherwise be required to file a tax return.

Re: I filed my 2020 taxes already and my federal was accepted. I received a form 1095-A after I filed

STOP ... DO NOT AMEND TO ADD THE 1095-A  ...

Wait for the IRS to ask for that form and the 8962  ... amending the return BEFORE the IRS asks for these items will delay the return even longer as the department which will ask for these things  and the department that processes amended returns are totally different and do not talk to each other.  AND a return cannot be amended before the original return is fully processed which will not happen with the missing 1095-A.  

If you have already filed your return and received an IRS letter, then you need to follow other steps.

1. Log into  your return
2. Select that you want to add a state - this lets you into the program without the amend process starting.
3. In the top right corner, locate the Magnifying glass/search  box 
4. enter the exact words 1095a
5. Select the first entry Jump to 1095a
6. Fill in the 1095a information
7. print and mail the 8962

{Edited 2/15/21 12:01pm}