Hi jindon, thanks for the answer!!. It does work, however the header is in row 6, data row 7 and goes from column "E" to "BA". I've tried changing the code but doesn´t seem to work. Could you check any mistakes please??

Private Sub Worksheet_Change(ByVal Target As Range)
    Dim r As Range, x As String
    If Intersect(Target, Columns("e:ba")) Is Nothing Then Exit Sub
    Application.EnableEvents = False
    For Each r In Intersect(Target, Columns("e:ba"))
        If r.Row > 1 Then
            With r.EntireRow.Range("d6").Validation
                .Delete
                x = Join(Filter(Evaluate("if(e" & r.Row & ":ba" & r.Row & "<>"""",e6:ba6)"), False, 0), ",")
                If Len(x) Then .Add Type:=3, Operator:=xlEqual, Formula1:=x
            End With
        End If
    Next
    Application.EnableEvents = True
End Sub
Also, some headers are combined cells (row 5 & 6) but not sure if that might be part of the error or not.