Be sure you need to amend. In order to have missed adding the credit to your return, you selected "yes" that you received the stimulus payment and then put in the amount you "received" in the box for the first round even though you did not receive it.
You should check line 30 on your copy of the form 1040 you filed. If you haven't already printed your copy, simply sign in and select the "Print/download" option. Here is a link that will help.
First allow the IRS to process your return. They may catch the error and correct it according to this link.
DO NOT file an amended tax return if you entered an incorrect amount for the 2020 Recovery Rebate Credit on your tax return. If you entered an amount on line 30 but made a mistake in calculating the amount, the IRS will calculate the correct amount of the 2020 Recovery Rebate Credit, make the correction to your tax return and continue processing your return. If a correction is needed, there may be a slight delay in processing your return and the IRS will send you a notice explaining any change made.
If the IRS does not correct the error and send the proper refund, you will need to amend your return. The amendment forms are projected to be available in TurboTax around 25 March. After that you can sign in to your account and select the "Amend" option. Follow the prompts to complete the changes. You will need to print, sign and mail in the amendments.