Hey Manager,
Thank you for the message.
This might occur due to certain limitations within Top Eleven's referrals system. While it allows Managers to link their club to a mentor and become their referral, it cannot be done multiple times from the same device, even when they are trying to do so with a new club.
If your brother would like to continue playing with the old account that's already connect to yours as a referral, he can try retrieving the access through a platform he might've connected it to (Facebook, Game Center, or Google Play).
Hope this helps! If you need any further help with referrals or account recovery, feel free to get in touch with our support team through the "?" in-game.
Thanks for playing!

Best, macmag123