Rails uses spring by default to speed up development. To run the generator, spring has to be stopped:
$ bundle exec spring stopRun shopify_app generator again.
App installation fails with 'The page you’re looking for could not be found' if the app was installed before
This issue can occur when the session (the model you set as ShopifyApp::SessionRepository.storage) isn't deleted when the user uninstalls your app. A possible fix for this is listening to the app/uninstalled webhook and deleting the corresponding session in the webhook handler.