You define a variable cl, but use cell in one line, so should be
Sub CountString()
Dim cl As Range
Dim rng1 As Range
Dim DD As Long
Set rng1 = Range("B2:B200")
For Each cl In rng1
If cl.EntireRow.Hidden = False Then
If cl.Value = "Ex lamp did not match Proposed." Then
On Error Resume Next
DD = Application.WorksheetFunction.CountIf(ActiveSheet.Cells, "Ex lamp did not match Proposed.")
End If
Worksheets("Chart&Calc").Range("K2") = DD
End If
Next cl
End Sub
but really you just need to add them
Sub CountString()
Const FILTER_VALUE As String = "Ex lamp did not match Proposed."
Dim cl As Range
Dim rng1 As Range
Dim DD As Long
Set rng1 = Range("B2:B200")
For Each cl In rng1
If cl.EntireRow.Hidden = False Then
If cl.Value = FILTER_VALUE Then
DD = DD + 1
End If
End If
Next cl
Worksheets("Chart&Calc").Range("K2") = DD
End Sub