Most of the time you wonder when working with Drupal rest APIs in your application and this error message creeps up 😀
Usually this error occurs when the client URL and server URL don’t match (including the port number)
So to resolve the problem we need to enable the service for CORS
If I understood it right you are doing an XMLHttpRequest to a different domain than your page is on. So the browser is blocking it as it usually allows a request in the same origin for security reasons. You need to do something different when you want to do a cross-domain request. A tutorial about how to achieve that is Using CORS.