Why can't I attack with my Great Dragon or Outpost Dragons?
If you are unable to use your Great Dragon or one of your Outpost Dragons when launching an attack, your Dragon is either healing or does not meet the necessary requirements to attack another player.
You will not be able to launch attacks with a Great Dragon or Outpost Dragon while they are healing. To see if your Dragon is healing, go to the City or Outpost it resides in.
You will need to wait until your Dragon fully recovers before you can launch a new attack with that same Dragon. To heal your Dragon more quickly you can always use Speed-Ups acquired through Fortuna's Chance or the Shop.
If you are still unable to attack with your Dragon and it is fully healed, be sure to check that you have the appropriate research and armor required to send each of your Dragons into battle. It is necessary to have Level 1 Aerial Combat (or higher) in addition to all four pieces of armor for your Great Dragon and Outpost Dragons to attack.