It is because you haven't qualified Cells(1, 1) with a worksheet object, and the same holds true for Cells(10, 2). For the code to work, it should look something like this:
Dim ws As Worksheet
Set ws = Sheets("SheetName")
Range(ws.Cells(1, 1), ws.Cells(10, 2)).ClearContents
Alternately:
With Sheets("SheetName")
Range(.Cells(1, 1), .Cells(10, 2)).ClearContents
End With
EDIT: The Range object will inherit the worksheet from the Cells objects when the code is run from a standard module or userform. If you are running the code from a worksheet code module, you will need to qualify Range also, like so:
ws.Range(ws.Cells(1, 1), ws.Cells(10, 2)).ClearContents
or
With Sheets("SheetName")
.Range(.Cells(1, 1), .Cells(10, 2)).ClearContents
End With