Поле Many2one (payment_tx_id) на Many2many (action_ids) тип изменен с Odoo12.
Попробуйте ввести следующий код:
order = request.website.sale_get_order()
#if you need to get last transaction
transaction = order.get_portal_last_transaction()
reference = transaction.reference
#if you need all reference to listed
reference = str(', '.join(order.transaction_ids.mapped('reference')))