You don't have a Sheet6! (I assumed you did since it was in your original code!)
Try this, adapting sheet names as necessary:
[VBA]
Dim varOne, varTwo
varOne = Application.InputBox(prompt:="Click first cell", Title:="Select 1st cell", Type:=8)
varTwo = Application.InputBox(prompt:="Click second cell", Title:="Select 2nd cell", Type:=8)
With Sheets("Sheet1")
If varOne <> False Then
.Cells(10, 8).Value = varOne
Else
.Cells(10, 8).Value = 0
End If
If varTwo <> False Then
.Cells(11, 8).Value = varTwo
Else
.Cells(11, 8).Value = 0
End If
.Cells(12, 8).FormulaR1C1 = "=Sum(R[-2]C:R[-1]C)"
End With
[/VBA]