You are attempting to use the GitLab auth backend, but it is not properly configured. To configure, please check the following:

• You have added {{ root_domain_uri }}/complete/gitlab/ as the callback URL in the OAuth application in GitLab. You can register OAuth apps at GitLab applications.
• You have set {{ client_id_key_name }} in {{ settings_path }} and social_auth_gitlab_secret in {{ secrets_path }} with the values from your OAuth application.
• Navigate back to the login page and attempt the GitLab auth flow again.