ERROR: (gcloud.app.deploy) INVALID_ARGUMENT: unable to resolve source

Try disabling and enabling the Cloud Build API in your project so a new service account is created, then try to deploy again.

This ensures that Cloud Build has permission to start builds (the service account name should look like [email protected]).

I found that it took a while for the API permissions to propagate fully.

I had this error after enabling the App Engine Admin API, but I waited a few minutes and tried again and it worked fine.