There can be several reasons you can not join a Clan:
1. You have not registered an account with your email address, and are playing Triviador as a Guest player. Guest players can not join a Clan.
2. You have not reached the necessary level requires by the Clan (You can find more information about the levels HERE).
3. You are trying to join Clan that requires an invitation.
You can find more information about Clans HERE.