This message usually disappear when your app finishes building and deployment. If the message persists, that means an error is preventing your app to run properly.
Error 502 means we are unable to route web traffic to your app. Either because it's still in building or deployment mode or because an error is preventing the app to start.
If you see this message after 2-3 minutes, check the Logs tab inside Settings. You can also ask the agent for help, tell him you have a 502 error.