Failed to establish communication with Looker host
When you build using the Extension Framework, Looker hosts your react application internally so you don't need to do authentication. Don't use the Extension Framework if you are not hosting your react application inside Looker itself.
If you're hosting your own application and making API calls to Looker then you should just use the Looker Embed SDK. You will need to do the authentication on your on webserver using nodejs / python / ruby etc. You shouldn't do authentication from client-side javascript.