Custom app integration

Question:
I have created a self hosted app on WIX studio and added App id and secret on the XPS shipping to integrate the shipping services. Now, even after doing everything by the documentation, it gives an error.

I see the app installed in my “Installed Apps” section, but when I click on it, nothing happens.
Product:
[Which editor or feature is your question most relevant to? e.g. Wix Editor, Wix Studio Editor.]

What are you trying to achieve:
Trying to integrate XPS Shipping app by creating a custom self hosted app.

What have you already tried:
Attached document

Additional information:
[Include any other pertinent details or information that might be helpful for people to know when trying to answer your question.]