I'm not sure where you are getting this error about not summarizing a field. I would create a formula and within it have a global variable. Put the formula in the Details section and have it increment when the condition is met. Then in the report footer (or wherever you want it), display the result of the formula. I've done this countless times, so maybe your circumstance is a bit unique? If so, please clarify a bit. It sounds like you already know CR well enough, so maybe this is something unique about your report?

CR performs calculations by their print order. So if you are doing a calculation in Section C and you are printing it in Section A, then it will be shown as zero in Section A. You can only display it in Section C or any section following it. What some people do to get around this is to create a subreport that just does that calculation (putting into a Shared variable) and putting that subreport in the report header. This makes the calculation available anywhere on the report (I talk about handling these 'hidden' calculations in Tutorial 9-6 of my new book).