"Since when does the IRS have to make sense? "

It's not the IRS I'm questioning here, it's Lacerte.  The F1120S instruction never say to file a 2020 statement on 2021.  It simply says do not file an amended return to attach a statement to 2020 if you claimed the tax-exempt income in 2020.  Yet in Lacerte you have to enter data for 2020 in order to clear the critical diagnostic.  That doesn't make sense.  I have a client that didn't receive any PPP money until 2021.