The federal form was rejected for my client [client name removed], but the form was not submitting before. The State of California accepted it not the federal.
Please remove your clients name from your public post.
So someone already filed a federal return for that SSN...maybe they used the IRS Non filer site to get the stimulus?
It sounds like either (a) you have the SSN wrong or (b) that SSN has already been used on a tax return.
If (a) correct and submit federal only and Call FTB to correct the accepted return.
If (b) you will need to send a paper return and warn the client there might have been identify theft.