After billing , it looping to cart page back. debug error message from chrome Jquery-3.3.1.min.js
Mixed Content the page at 'https://www.nop.com/onepagecheckout' was loaded over HTTPS, but requested jquery.3.3.1min.js an insecure XMLHttpRequest endpoint 'http://www.nop.com/checkout/OpcsaveBilling'. The request has been blocked; the content must be served over HTTPS