This application does not exist (app_id=xxx)
What worked for me is to use the project id from the google app engine console instead of the project name in app.yaml
I was unable to upload to an AppEngine as appcfg was telling me :

This application does not exist (app_id=u'xxx').

I was only a developer on the AppEngine, so as I was just testing I created a new AppEngine where I was the owner but I still get the same message on a newly created AppEngine.