Sum Lastrow formula!!

No, just the activesheet, for all sheets I would do that like this:

Sub sum_C_column()
Dim wSh
For Each wSh In ActiveWorkbook.Sheets
wSh.Range("C" & Cells(Rows.Count, 3).End(xlUp).Row) = Application.WorksheetFunction.Sum(wSh.Range("C1:C" & Cells(Rows.Count, 3).End(xlUp).Row))
Next
End Sub

Edit - I've tested all the code I've put and it all works without error on my setup. Could you upload a workbook with the code as you are getting an error?