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 and badge! 😥
• Choose its badge and enter your team description.
• Choose your team type :
  • Open: In an “Open Team”, a player can join the team at any time, as long as they meet the level requirements.
  • Closed: On a “Closed Team” a player should be approved by the team leader in order to join the team.
• 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 100 in-game coins 💰