Why can't I access my brother's Facebook account?

[Marwan]

Maybe he blocked you and set his Privacy settings so that no one finds him except his friends.

He may have set his privacy settings like this:-
Account ---> Privacy settings ----> Basic Directory Information ---> View settings ----> Search for me on Facebook ---> Friends only.

Click the "Preview My Profile" button

Account ---> Privacy settings ----> At the bottom --->Applications and Websites
----> Edit your settings ---->Public search ----> Edit settings ---> Leave the box "Empty" Unchecked

A block prevents specific people from viewing your profile. Any ties you currently have with the people you block will be broken (friendship connections, friend details, etc.). Your profile will not be visible to them and you will not appear in their search results or friend lists. Blocking is mutual, so they will also become invisible to you as well.