You have to select the sheet before you can select the range.
I've simplified the example to isolate the problem. Try this:
Option Explicit
Sub RangeError()
Dim sourceBook As Workbook
Dim sourceSheet As Worksheet
Dim sourceSheetSum As Worksheet
Set sourceBook = ActiveWorkbook
Set sourceSheet = sourceBook.Sheets("Sheet1")
Set sourceSheetSum = sourceBook.Sheets("Sheet2")
sourceSheetSum.Select
sourceSheetSum.Range("C3").Select 'THIS IS THE PROBLEM LINE
End Sub
Replace Sheet1 and Sheet2 with your sheet names.
IMPORTANT NOTE: Using Variants is dangerous and can lead to difficult-to-kill bugs. Use them only if you have a very specific reason for doing so.