Hi,

Purchase order approval means setting limits for the purchase of goods by a company's employees. The manager can set a minimum amount as the limit for the purchase. After this, when the total purchase amount exceeds the limit, the employee needs approval from the manager to confirm that purchase order.
Does not have a built-in Sales Order Approval feature similar to the Purchase Order Approval.
Third-party modules available that add Sales Order Approval functionality to Odoo ,Please check this:  https://apps.odoo.com/apps/modules/15.0/sales_order_double_approval/

Hope it helps