(Paginated Report/ErrorCode: rsFieldReference) Getting COUNTROWS of different dataset from a tablix

Bit boggled by this, I'm trying to get the COUNTROWS of a separate dataset from a tablix but keep running into the rsFieldReference error:

The Value expression for the text box 'Textbox21' refers to the field 'DateStart'.  Report item expressions can only refer to fields within the current dataset scope or, if inside an aggregate, the specified dataset scope. Letters in the names of fields must use the correct case.

This is the formula in question:

=SUM(
IIF(CDATE(Fields!Date.Value) >= CDATE(Fields!DateStart.Value) AND CDATE(Fields!Date.Value) <= CDATE(Fields!DateEnd.Value), 1, 0),
"dsHolidayData")

Any way I can resolve this? The main dataset I'm pulling the DateStart/DateEnd dates is a SQL query and the dsHolidayData dataset is from a PBI dataset connection so I don't think there's a way to do this via a query.