There are issues with specific Facebook accounts that prevent you from linking.
Here are some of the possible solutions to be able to link your account successfully;
• If a Facebook profile is already linked to Brave Frontier Account, please use a new one.
(same use of one facebook account is not allowed)
• Check if your Facebook profile is currently logged-in in your Facebook App
• If you recently changed password, please update it in your Facebook app
• Allow Brave Frontier to gain access to Facebook (For iOS Users)
• Re-log your Facebook account
• Restart your device.
If these known methods do not work for you then we advise checking the technical section in this site. If you can not find the solution then please do send us a message via our Game Support.