A function is not a property / method out of range.
If you want to summarize the values, use the following:
Range("A1").Value = Application.Sum(Range(Cells(2, 1), Cells(3, 2)))
EDIT:
if you want the formula to be used 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).
If you always use the same range address, you can use it as a Rory:
Range("A1").Formula ="=Sum(A2:B3)"
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