I got the same issue and finally solved it after 2 hours.
I installed ngrok separately on windows, ran the command to add the auth token to the default configuration file, plus the command to add my token for the session, still didn’t work. So the problem is that after running “npx @mondaydotcomorg/monday-cli scaffold run ./ quickstart-react”, the ngrok url it outputs for you is incorrect. Stop running the program now. Run “npm start”. It starts the program now. Then in your browser, type “http://localhost:4040/status”, and right under the “command_line” section, copy paste the url. You can open that url in a new tab and see you app running. And copy that url into the monday.com app url. For some reason the actual ngrok url is different than what the original output says. Or there’s some funky authentication issue. Idk why running the program with npx, makes the url live, but doesn’t recognize your credentials. So you gotta start it normally with npm to fix that issue.
@alessandra I spent 2 hours trying to figure out the issue and only now did I find it. If you could escalate this and get the monday.com devs to update the guide as multiple people are experiencing it, that would be great.