In addition to what Sinisa wrote; apps might require additional scopes that are necessary for them to work properly. Those scopes are defined in app.config.ts. Here's an Automations app example:
More details about the scopes setting: https://developer.dynatrace.com/reference/app-toolkit/configuration/#app-options
That's what I needed! I had to just add:
@sinisa_zubic any plans to add it to the Dynatrace Developer portal?
it is already in Dynatrace Developer in the SDK documentation. e.g. when you use the getCredentials function you can see the needed scope: https://developer.dynatrace.com/reference/sdks/client-classic-environment-v2/#getcredentials
also you see for which call you need the app-engine:apps:run scope, e.g. here: https://developer.dynatrace.com/reference/sdks/client-app-engine-registry/#getapp
There are ideas for improvement, but no timeline