Solution found:
As noted in the original post, it would appear the data should be able to be entered on 1040/1040SR Wks, which has a blue box which, when I've seen it elsewhere, I have been able to enter data. It does not. So I chose to "Override" (even though there's no data entered to override) and entered the payment that way. No errors resulted.
Why the data entry field fails to allow the entry of data on this worksheet, but requires an override, is a mystery, like so many things with Intuit products.
