I see you are filling Proxy server field with the ThingWorx Experience Service URL, which is not correct.
The proxy server and user credential fields are the ones where the details of your NETWORK proxy should be provided. The Proxy server provided in Studio settings pop-up is no different from the actual proxy server
To publish your experience, please do clean the Proxy URL, username and password, and try publishing the experience. Still you see the same problem, then it should be your network security issue i.e., your corporate PRIVATE network doesn't allow request-response from ThingWorx Experience Service URL. In that case, I would recommend using PUBLIC network or a Non-restricted network to publish the experience.