Can I stop a return that has not been accepted yet?

[DoninGA]

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

You will have to wait for the IRS to start accepting or rejecting 2020 tax returns on February 12, 2021
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 1040X, can only be printed and mailed to the IRS, it cannot be e-filed. The IRS will take up to 16 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 on amending a tax return originally completed and filed using the 2020 TurboTax online editions - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/01/27577

[VolvoGirl]

No.   You cannot cancel or change or add anything on the return that you e-filed.  It is too late, just like when you put an envelope in a US mailbox

Now you have to wait until the IRS either rejects or accepts your return. Which won't be until Feb 12 this year. If your return is rejected, you will be able to go into your account and add or change it and resend it. Click on Add a State to let you back into your return..

If the IRS accepts your return, then you have to wait until it has been fully processed and you have received your refund or paid the tax due. THEN you can prepare an amended tax return 1040X and mail it in. You cannot e-file an amended return.  Meanwhile, DO NOT change anything on your return until you start the amended return. If you did change or add the missing info then take it out and put your return back to the way it was when you first filed it.

You can check your efiling status here,
https://turbotax.intuit.com/tax-tools/efile-status-lookup/

How to amend an Online return
https://ttlc.intuit.com/community/amending/help/how-do-i-amend-my-turbotax-online-return/00/27577