I'm getting this error when trying to impersonate a user of a domain whose admin installed my app from google apps marketplace.
The code I'm running:
credentials = service_account.ServiceAccountCredentials.from_json_keyfile_dict(key_data, scopes=REQUIRED_SCOPES) # The key data is generated when creating a service account from my project.
creds = credentials.create_delegated('user1@my_domain.com')
http_client = creds.authorize(Http())
discovery.build('gmail', 'v1', http=http_client)