We need to generate secret key for development environment.
mix phoenix.gen.secret
# ednkXywWll1d2svDEpbA39R5kfkc9l96j0+u7A8MgKM+pbwbeDsuYB8MP2WUW1hfGuardian requires serializer for JWT token generation, so we need to create it lib/my_app_name/token_serialize.ex. You need to restart your server, after adding files to lib folder.