To get Observable to work with FHIR you need ..
FHIR and Observable
This will give you a url you can register as your redirect url for the app
Cerner code Console
Log into code-console.cerner.com
Click Log in
Create a new App
Give it a name, make it a patient access app, online, public, and give it the url of your Observable notebook.
Use R4 APIs (they will be around longer than DSTU2)