Is there anyone else who could have filed using him as a dependent? Is he old enough to be working and file his own taxes? Are you sure you are entering his proper SSN?
Yikes! Possible identity theft! You need to notify the SSA (Social Security Administration) and the IRS right away that someone else might be using your SSN. I've been told that your should also report it immediately to your local police/sheriff department.
The most common reason for this to occur for tax year 2019 is when a taxpayer has incorrectly filled out the nonfiler form for the economic impact payment ("stimulus check") previously.
Other reasons can include being listed as a dependent of another taxpayer or, occasionally, identity theft.
There is no workaround for this error. The return must be printed and filed by mail.
Did you file for a stimulus payment earlier? If a tax return or stimulus registration was already done with your social security number, you have to print and mail your return.
The Economic Impact Payment’s Non-Filers tool was not intended for use by taxpayers who also needed to file a tax return with the IRS for tax year 2019. If you used the Non-Filers tool to register for an Economic Impact Payment, you cannot now file your tax return electronically.
You must complete and print a paper 2019 Form 1040 or 1040-SR tax return, write “Amended EIP Return” at the top, and mail it to the IRS. If you owe tax, you should pay it in full by the July 15 due date. Interest will be charged after July 15 on any amounts that are not paid by July 15. For more information, visit the Amended EIP Return page.