Fill in drop down box when item entered cell

Right Mouse-Click sheet tab and select "View Code". Put this code in the window that opens.

Private Sub Worksheet_Change(ByVal Target As Range)
    Dim rCheck as Range, rTest as Range

    On Error Goto EF
    Application.EnableEvents = False
 
    Set rCheck = Range("A" & Target.Row)
    Set rTest = Range("D" & Target.Row)

    If rCheck.Value = "" Then
        If rTest.Value <> "" Then
            rTest.Value = ""
        End If
    ElseIf rTest.Value = "" Then
        rTest.Value = "Not Tested"
    End If

EF:
    Application.EnableEvents = True
End Sub