The fix to this issue is because the form key validation is missing on check out. You can view the fix here How to fix Magento checkout after upgradingHow to fix Deprecated Functionality: Function mcrypt_module_open() is deprecated in Magento 2Want to know when the Magento platform started?
The fix to this issue is because the form key validation is missing on check out. You can view the fix here How to fix Magento checkout after upgrading
