Sum ( 5 ) produces the total 5.
WHen you use Sum ( fieldName ), you are doing the same thing.
Sum needs a list of values to sum in one of the following formats:
Sum ( field1 ; field2 ; field3 ) // individual fields in the same record
Sum ( RepeatingField ) // repetitions of single field are summed
Sum (RelatedTable::Field ) // Value of field from all related records are summed
None of these would seem to quite do what you want here, though the third option can be set up with a special self join relationship to do that.
But there's a simpler option: Add a field of type Summary instead of a calculation field. Specfiy that it compute the "total of" the field you want to total.
Note: This computes the total for all records in your current found set--to get a total of all records in your table requires putting all records in your found set--which can be done with Show All Records. If you need a total of all records in the table when you don't have all records in your found set, you'll need to use the "self join relationship" method I referred to earlier. Respond back here if that's the case and I'll respond with a detailed description of how you can set that up.
Create a sumary field instead of a calculation field.