If you are getting infinite loading animation, usually it means that you have some critical issue on the backend side. First of all, you need to check the error details. For this purpose, check your log files:
- webserver error.log
Or you can enable developer mode for Magento 2 in your shell:
php bin/magento deploy:mode:set developer
After this you should have a detailed error trace right in the response body of your order place request. Just check the response body of the place order request in your browser right after clicking the "Place Order" button (in Chrome you can track all requests in the Network tab of the developer tools pane).