Hi Michal,
Could you clarify if you have deployed your WordPress app on App Engine?
If you have deployed your app in App Engine, you may follow the steps provided in “Mapping Custom Domains” and “Securing Custom Domains with SSL” documentations to double-check the steps you have taken to add your custom domain and SSL certificate to your app.
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The aforementioned documentation I noted, shows how to add your custom domain and SSL certificate for your App Engine app. The error you mentioned indicates that your don’t have permission to access the app or it may not exist.
It seems you don’t have any App Engine app in your project. If so, I recommend to take a look at this thread in Stack Overflow. If the issue still persists, you may post your full detailed question to Stack Overflow, since Google Groups are reserved for general Google Cloud Platform-end product discussions and not for technical questions.