SSRS error - A scope is required for all aggregates in the page header or footer which reference fields
The reason I was getting the error message is because I was supplying the scope (dataset) in the wrong place.

The scope needs to be supplied as the second argument of the aggregate function. I was incorrrectly supplying the scope as part of the left function (which is not an aggregate function) instead of the SUM function.

Below is what the working expression looks like. The only difference from my original syntax is the location of the scope/dataset name (PPR) in the expression.

="Interim Score:   " &amp; CINT((sum(CINT(left(fields!Points.value,1)),"PPR") / sum(CINT(right(fields!Points.value,1)),"PPR"))*100) &amp; "%"