I am facing strange issue on my Odoo 11 community. When i try to add order line in my existing quotation or sale order it give me access error (Document type: sale.order.line, Operation: read).
But when i remove all order line then when I try to add order lines again I have no error. Every time I have to edit my quotation or Sale order and add new order line I have to delete all the existing order lines and add them again with the new one. This is troublesome when I have more than 5 order line because I have remember all of my previous order line.
This is not a standard Odoo behaviour and your question can not be answered without the necessary information about your customizations/addons.
@Ermin Trevisan
I think it has nothing to do with Customization. Everything is stock with only field addition.