Castle Crush's subscription is a new feature and it's currently not available for all players at the same time. In order to subscribe, the subscription offer must be available to you inside the game store. If you are unable to see the subscription offer in the game store, it means that the subscription is not available for you yet.

It works the same way for the Free Trial. However, in order to acquire a Free Trial, you must unlock the offer first. In order to unlock the Free Trial offer, it must be available to you in the game store. Once you receive the Free Trial offer, you must win 3 battles with a Flawless Victory! After that, the Free Trial will be unlocked and you get 3 days of free subscription. After using the 3 days for free, the subscription will automatically renew and become a Weekly subscription. If you don't want to be charged, you must cancel the Free Trial at least 24 hours before the renewal date.

App stores can only process a subscription purchase if there is a credit card on file in your account. A credit card is needed because the subscription will be recurrently charged and will automatically renew. So if you want to purchase a subscription it is necessary to have a valid credit card registered in your account.