A 19 year old child can qualify to be your dependent if they meet all the rules to be a dependent. It sounds like you entered all the data in to TurboTax, determined your child met the rules to be your dependent, filed your return and then your return was rejected by the IRS.
In order to fully answer your question, I would need to know the specifics of the reject; but without that, I can try to explain what probably happened.
The IRS electronic filing system will reject your return if it includes a dependent that either was already claimed as a dependent by another electronically filed return, or that the dependent filed a return and did NOT indicate on the return that they CAN be claimed as a dependent.
So, in your situation, one of two things happened. Another person may have filed a tax return and listed your son as their dependent on their tax return - and if they filed first, your return would be subsequently rejected, as that social security number has already been used. A reject does NOT mean your return is wrong and theirs is right, it just means the second return cannot be electronically filed. If this is the case, you would have to paper file your tax return. At some point of time down the line, the IRS will follow up via IRS notices with you and follow up with the other person to determine which return was correct and which one was not correct.
The other situation that often occurs, and I am guessing this is the case for you as your child is 19 - is that your child may have filed their own tax return, but filed it INCORRECTLY. When a person who is a dependent files their own tax return, there is a check box on the return that asks "CAN you be claimed as a dependent by someone else". If you meet the rules to be considered a dependent of someone else, you are required to check that box. I suspect that your child may have filed their own tax return and did NOT indicated that on the return, meaning their return said they are NOT a dependent, so then your return cannot be electronically filed claiming them as a dependent.
If this is your situation - the resolution is that you will have to paper file your tax return - and your child should amend their tax return to show that they can be claimed as a dependent by someone else.
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