Frustrated-in-IL,

Wanted to hop in and explain why you are seeing that error message in the product that is blocking you from filing a short year return with a 11/30/23 year end in the 2023 Proseries software.  

On page 1 of the instructions for the IL-1120ST, Illinois specifically identifies that only filers with a year ending on or after 12/31/23 and before 12/31/24 may file on the 2023  version of the forms.

You may want to consider using the 2022 software to file electronically with a 11/30/23 year end.  See screenshot from the instructions for more detail.  

I hope this is helpful.