Despite 3 hours on the phone with TurboTax support not resulting in finding a solution, further experimentation on forms resulted in my being able to find an entry point for this necessary information and completion of the error checking process. This is clearly a bug in the program and wasted a ton of my time.
To get around the limitation of the step-by-step process, change to Forms (upper right of program window). Go to "Sch M-1 Wks" and look for the number without a description; here it will allow you to make an entry, and the exclamation point next to Form 1120S p. 3-5 will disappear. You can now complete error checking and file electronically - until I reached this point it was not apparent whether or not I could do that, and Intuit support suggested that I might need to print and file by mail.