Callback URL should be the same as the domain URL in your config.json file with the callback addition.
Config File = *https://*example.com
Discord Dev Portal = *https://*example.com/auth/discord/callback
Make sure the domain element in your config file does not END with a /
Correct = https://example.com
Incorrect = https://example.com/
Also make sure your Client Secret, Client ID, and Bot token ALL MATCH THE APPLICATION YOU ARE SETTING THIS UP IN!