One way:
Public Sub NT_comment()
Dim c_ell As Range, Date_Ref As Date, S_heet As Worksheet, c_ell2 As Range
For Each S_heet In ActiveWorkbook.Sheets
If Left(S_heet.Name, 3) = "ACD" Then
S_heet.Select
For Each c_ell In Range("D2", Cells(Rows.Count, 4).End(xlUp))
Select Case c_ell.Value
Case "C to BBNT": c_ell.Offset(0, 1) = "OK"
Case "Will C SD": c_ell.Offset(0, 1) = "OK"
Case Else: c_ell.Offset(0, 1) = "Not OK"
End Select
Next c_ell
End If
Next S_heet
End Sub
Think you were specifying the wrong column [Cells(Rows.Count, 4).End(xlUp))]
Regards
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