If your order/checkout page always displays “cart is empty” or similar, even after you have added item to the cart in other pages
it is usually due to the required WPPizza orders database table not being created because the site is using an old mySql version.

WPPizza requires mySql 5.5+ – (see requirements: https://wordpress.org/plugins/wppizza/ – In fact, WordPress itself recommends 5.6+ )

If the above is the case, update your version of mySql or get your host to update it for you. Once you have done so go to "Wppizza -> Tools -> Maintenance [Tab]" enable/check "Update order table" and save once.