I think you are misinterpreting the source of the error; rExternalTotal appears to be equal to a single cell.
rReportData.offset(0,0) is equal to rReportData
rReportData.offset(261,0).end(xlUp) is likely also equal to rReportData, as you offset by 261 rows and then use the .end(xlUp) function which selects the top of a contiguous data range.
If you are interested in the sum of just a column, you can just refer to the whole column:
dExternalTotal = Application.WorksheetFunction.Sum(columns("A:A"))
or
dExternalTotal = Application.WorksheetFunction.Sum(columns((rReportData.column))
The worksheet function sum will correctly ignore blank spaces.
Let me know if this helps!