Error 1: "Unfortunately an error has occurred and your information has not been saved."
1) Check the Address Book Mode for the site in Sites > Settings > General Info: Checkout > Address Book Mode. If it is set to "Locations" or "Both", make sure you have at least one Location added in Sites > Locations & Departments.
2) Make sure you did not double-click the checkout button. Doing this causes the first click to submit the order, and the second click to attempt to submit an order of an empty shopping cart.
Error 2: "Shopping cart is not ready for checkout."
There may be a conflict between Site > Setting and Site > User .
Check if Site > Setting > General Info tab > Checkout has Purchase Order set to Required. Check if Site > User > General Info tab > Added Info has Purchase Order set to Do Not Ask. These two settings conflict. If the user must enter a PO to check out, but their user account is set to not ask for one, they are stuck and the above error will display. Change the Site > User setting to "Use Site Setting" in this case.