If even one of the 5 possible raiding parties is made up of a combination of Heroes that is subject to limitations during Chaos season, you cannot start a battle.
The following conditions apply to raiding parties during Chaos season.
・ There cannot be more than one of the same Hero.
・ There can only be one Hero with a Sing or Dance skill per raiding party.
・ There can only be one Hero with a Savior skill per raiding party.
Please check to see if all of your Chaos season raiding parties fulfill the above conditions.