else without if

Try:

If Cells(6, 18) = 0 Then
    Cells(4, 20).Select
Else
    If Cells(6, 18) = 5 Then
        Cells(5, 20).Select
    Else
        If Cells(6, 18) = 10 Then
            Cells(6, 20).Select
        Else
            If Cells(6, 18) = 15 Then
                Cells(7, 20).Select
            End If
        End If
    End If
End If

But you should avoid the Selects

Regards