Hello again
I found such a solution and it works:
If Me.x > DLookup("[stock_level]", "tbl_stock", "[Warehouse_ID] = " & Forms!frm_stock!series_product ) Then
MsgBox "x is to big", vbInformation, "Stock"
Me.x.SetFocus
I think they referred to the wrong cell in the table.
now looks up...