Call the number on the reject notice.  They might be able to provide some info.  You will need the letter your client received from the IRS, approving the S election.

Hopefully, that letter exists.  I have one that the attorney faxed the election to the IRS, but the approval seems to be MIA.  I have another one that the attorney never filed the election, and is just now saying 'hey, it wasn't our responsibility'.

On both of them, the 2022 returns went thru as efiled, but the 2023 returns were rejected.  One just got a notice from the IRS that 'oops' the 2022 return really shouldn't have been a 1120-S.

I would bet your client is in the same situation. 

It's a mess.