When you run your web app in Chromebox, you cannot call out the DevTools to check the error in console or inspect elements. Instead, there is another way to do so.
Firstly, do not run web app in kiosk mode, and quit fullscreen mode. Open Chrome browser, in address bar, type: chrome://inspect
On left menu, click Apps, you will find the web app you are running, click inspect to call out the DevTools for that web app.
Then you can do your debugging as usual.