It seems that you haven't set up discord 0auth correctly, but fear not, the docs come to the rescue! https://CtrlPanel.gg/docs/Installation/additional-configuration#discord-auth
Extra information about URL Formatting
Any URL saved in your settings should never end with a /slash; CtrlPanel adds this / automatically to any URL, adding an extra
/ will break the URL!
GOOD Example:
https://CtrlPanel.gg
BAD EXAMPLE:
https://CtrlPanel.gg/
When configuring discord-auth, we will automatically use your APP_URL to create the appropriate URLs needed for discord-0auth. Make sure your URLs are correct!
When using SSL-Encryption (https://) double-check that all your URLs start with https:// instead of http://
After editing the .env File you have to clear the Cache:
php artisan cache:clear
php artisan config:clear