Hi,

This o_wsale_product_page class is not present in odoo15. This is the extra class present in odoo16 website_sale --> views --> template.xml --> . When you are changing from odoo15 to odoo16 you have to add this class in div having the class name="js_sale". 

This is important as there are many templates in website_sale template.xml which are inheriting this template having id="product" by using this class name o_wsale_product_page.