How to participate in clan war instead of being a spectator

I joined a new clan and now that the clan war has started I tried to attack but it says "Spectator can't attack". How do I get out of being a spectator (I haven't used either of my 2 attacks in the war). I went to the clan castle and the clan option and I selected "Yes I'll participate in this war" but nothing happens.

You have probably started the clan in the middle of a battle, meaning you have to wait this one out. However it may be that your leader decided he didnt want you in the war, which he can choose at will.

The participants in a war are by multiples of 5. If a clan has 19 people whith greenlight for war, only 15 people will actually participate and the 4 others will just watch. I am not aware though on which criteria each participants are selected.