In order to start a Clan War, you need at least 5 people in the Clan.
Note that you can only register for the current Clan War during a limited period of time each week. The countdown is shown right next to the Clan War button. If you register for the Clan War too late, you may experience extended waiting times to find opponents or potentially not find any opponents at all.
If all conditions are met for the Clan War to start, but it has not started yet (you see a grey button "Waiting for Opponents"), then try to wait for 5-10 minutes or restart the game.
Should the issue persist, please contact our Support Team. Provide your in-game ID, your Clan’s ID, and a screenshot of your Clan Lobby showing the Clan War button. It is also helpful if you ask other Clan members whether this issue is affecting them as well.