What does it say on the letter? https://www.irs.gov/pub/notices/cp261_english.pdf
OR Read the instructions if you are applying: https://www.irs.gov/instructions/i2553#idm139855003296080
Not a letter. On the return itself.
@Roz1 What date is on the letter IRS sent to the S-Corp? I gave you a sample letter link, now look at the actual letter that your client has.
What does the error message say?
Is the S-election date on or after the incorporation date?
Is the S-election date the on the first day of the tax year for the corporation?