Flask-OAuthlib OAuth2 client Error: "Missing access credentials."
You need to define a scope too: for example:
request_token_params={ 'scope': [ 'https://mail.google.com/', 'https://www.googleapis.com/auth/admin.directory.user.readonly', 'https://www.googleapis.com/auth/admin.directory.orgunit.readonly', 'https://www.googleapis.com/auth/admin.directory.group.readonly', 'https://www.googleapis.com/auth/userinfo.email', 'https://www.googleapis.com/auth/userinfo.profile', 'https://www.googleapis.com/auth/plus.profile.emails.read',]