If you used the IRS "non-filer" tool to register for the stimulus, that counts as a tax return (you were telling them officially that you were not planning to file a return.) You must now print your return and file by mail. Write "Amended EIP return" at the top of page 1.
If you did not use the non-filer tool to register for the stimulus, it is possible that you are the victim of identity theft. You will also have to print your return and mail it in, and wait for the IRS to contact you for more information. You should also check your credit reports for any other evidence of identity theft fraud against you, and you may want to lock or freeze your credit reports to prevent someone from opening new accounts in your name.
