You can only create a team if you're currently not on any other team:

Open the team window and go to the Create team tab.

- Enter your team name. This field is mandatory. 
- Once your team is created, you can't change its name! 😥
  Enter your team description, choose its badge and type.
  Set the required level for entry. Players who are below that level in the game won't be able to join your team. The required level can't be higher than your own level in the game.
- Tap the Create button. Creating a team costs 150 in-game coins 💰