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

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

• Enter your guild name. This field is mandatory.
• Once your guild is created, you can't change its name! 😥
  Enter your guild 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 guild. The required level can't be higher than your own level in the game.
• Tap the Create button. Creating a guild costs 100 in-game coins 💰