Function is not a property/method from range.
If you want to sum values then use the following:
Range("A1").Value = Application.Sum(Range(Cells(2, 1), Cells(3, 2)))
EDIT:
if you want the formula then use as follows:
Range("A1").Formula = "=SUM(" & Range(Cells(2, 1), Cells(3, 2)).Address(False, False) & ")"
'The two false after Adress is to define the address as relative (A2:B3).
'If you omit the parenthesis clause or write True instead, you can set the address
'as absolute ($A$2:$B$3).
In case you are allways going to use the same range address then you can use as Rory sugested:
Range("A1").Formula ="=Sum(A2:B3)"
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