From the official portal.
At this time, the Ranked Team Battles game mode is permanently disabled in the NA region. You can still create a team in Unranked Team Battles, where you’ll face rival teams.
To play unranked, log into World of Tanks, open the dropdown menu to the right of the “Battle!” button and choose “Team Battle.” Under “Unranked Battles,” click “Proceed” to create or join a team.