What should I do if I accidentally selected that I did not receive stimulus payment?

Do nothing right now.  They will have a plan to reconcile the payments against the return that is filed.

If your return gets rejected, you will be able to fix it and then resubmit it immediately.

If your return gets accepted as filed, wait until it is fully processed before attempting to file an amendment.  It's quite likely the IRS will adjust the refund before your receive it.

If you find that you need to amend because they actually released the amount of the credit accidentally (not likely) then you can amend the return.

First check Get My Payment - Select the button Get My Payment to see if your information is current based on the payments you did receive. If so, then it's another confirmation the IRS will not release the funds a second time.

Lastly, if necessary you can follow the steps in this link to amend your tax return, it will be available March 25th. 

• How to amend (change or correct) a return already filed (and accepted at the IRS)