I forgot to submit form 8936 but already submitted my taxes.

[DoninGA]

Once a tax return has been filed it cannot be changed, canceled or retrieved

You will have to wait for the IRS to either Accept or Reject your filed federal tax return.
If the return is rejected you can make the necessary changes and e-file again.
If the return is accepted you will have to amend your original tax return. An amended return, Form 1040-X, can only be printed and mailed to the IRS if the original tax return was not e-filed. The IRS will take up to 20 weeks or longer to process an amended tax return.

Before starting to amend the tax return, wait for the tax refund to be received or the taxes due to be paid and processed by the IRS.

See this TurboTax support FAQ for amending a current year tax return - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-federal-tax-return-curr...

[DawnC]

Do you need to take the clean vehicle credit?   If so, wait until your original return is fully processed and you can file an amendment and add the tax credit.

If you transferred your credit at the time of a sale and need to add Form 8936, you should also wait to file an amendment.   The IRS may request you upload the form without filing the amendment.  In either case, you can add Form 8936 to your return when you amend in TurboTax.   To find the clean vehicles section in TurboTax:

1. Open or continue your return in TurboTax by selecting the Continue or Start button on the Tax Home screen.
2. Select Search, and enter Form 8936 in the search bar.
3. Select Jump to Form 8936 from the results.

How do I amend my federal tax return for this year?