Sage 300 CRE's Report Designer can be confusing enough, but the aggregating functions (aka "accumulating functions") in formulas are probably the most frustrating part to me. It's not that they're necessarily complicated in themselves. They're more complicated than an Excel SUM() or COUNT(), but they're not beyond understanding. It's the way they interact with a somewhat hidden setting that makes them frustrating.
They all follow the same formula: [FUNCTION NAME] ( [BREAK FIELD] , [FIELD TO AGGREGATE] ). If I want to get a sum of AP Payments, I'll use ASUM(), and I'll use "Amount Paid" from "AP Payment" as the field to aggregate. So a simple version looks like "ASUM(, Amount Paid)".
The Break Field complicates things, though. If you get that wrong, you'll get the dreaded "--" as a result. The Break Field is saying "I want to calculate this value for all records in this report that match each other on this field". It has to match the summary level for the Print Control setting you're using. Using the above example, I probably don't just want the total amount paid, I want the amount paid per invoice. To do that I need to make sure that the formula is "ASUM(Invoice, Amount Paid)", and I need to make sure that the print control is set to "Print Once Per Invoice".
To see the print control settings, you need to click on "View" and choose "Print Control". To change the print control settings, you select the field you want and click on the status bar in the bottom left of the report design window where it says "Print: ..."