The only time I get that error is when the sheet is still protected. However
if the password you have supplied is incorrect then the error should occur on
the unprotect line.

Also you should probably disable events here so that you don't get into a
loop. Do you want to make C6 the only cell which can be active on the sheet?
This is currently the case.

Finally, are you sure this is the formula you are wanting to place is C6:
=SUM(C7,1)

Maybe your event should look like this:

Private Sub Worksheet_SelectionChange(ByVal Target As Range)

On Error GoTo ErrorHandler
Application.EnableEvents = False
ActiveSheet.Unprotect Password:="peterke"
Range("C6").FormulaR1C1 = "=SUM(R[1]C,1)" 'supply correct formula
ActiveSheet.Protect Password:="peterke"

ErrorHandler:
Application.EnableEvents = True
End Sub

Hope this helps
Rowan

"Peterke" wrote:

Hi,

Why does the error 1004 occure here?
(error marked between <<)
and what do I do to avoid it in the future?

Private Sub Worksheet_SelectionChange(ByVal Target As Range)
ActiveSheet.Unprotect Password:="peterke"
Application.ScreenUpdating = False
Range("C6").Select
ActiveCell.FormulaR1C1 = "=SUM(R[1]C,1)"<<
Range("C6").Select
ActiveSheet.Protect Password:="peterke"
Application.ScreenUpdating = True
End Sub


greets,
Peterke