422 Unprocessable Entity when Calling secure API with JWT using client.request()

We face an obstacle when integrating the authenticate that Zendesk support suggest for custom apps. We always getting 422 error code (Unprocessable Entity) with any api we call when enabling the secure:true which is documented in zendesk support website. Does someone has an explanation why this is happening?

The error message is: “failed to get installation and oauth information for app”

https://developer.zendesk.com/documentation/apps/app-developer-guide/using-the-apps-framework/#encoding-and-sending-json-web-tokens

We understand that you can't get the authenticated user token within the Custom App, currently the App framework's client.request() method supports supports encoding and sending JSON Web Token (JWT) that should suffice to authenticate and authorize the Agent on your back-end.