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

• You have registered {{ root_domain_url }}/complete/apple/ as the callback URL for your Services ID in Apple’s developer console. You can enable “Sign in with Apple” for an app at Certificates, Identifiers & Profiles.
• You have set SOCIAL_AUTH_APPLE_SERVICES_ID, SOCIAL_AUTH_APPLE_APP_ID, SOCIAL_AUTH_APPLE_TEAM, and SOCIAL_AUTH_APPLE_KEY in {{ settings_path }} and stored the private key provided by Apple at /etc/zulip/apple-auth-key.p8 on the Zulip server, with proper permissions set.
• Navigate back to the login page and attempt the “Sign in with Apple” flow again.