I submitted to the IRS that I had not received the third stimulus payment in 2021. As it turns out, my parents claimed me. How do I specifically amend this error?

It depends. Has your return been accepted yet? If your return has been accepted then there is no need to amend your return just to fix the third stimulus payment input, since the IRS will check your response against their own records and will adjust your refund amount accordingly. However, due to this your refund will get delayed since the IRS will adjust your tax return which will delay the process.

If your return gets rejected, you can simply go back to your return and edit your response.