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

• Tap the Teams icon 👥 in the bottom-right corner of the game screen to open the Teams window. Then go to the Create tab.
• Enter your team name. This field is mandatory. Once your team is created, you can't change its name!
• Enter your team description and choose the logo.
• 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 diamonds 💎