Events are time-limited challenges where you can play single or a set of missions in order to receive unique rewards.

Some events allow you to compete with other players for top position of the leaderboard. Other events types challenge only your skills. And to play Gang Events, you need to be in a Gang.

Events require Energy as well as added mission conditions (certain item types, item gear score limits, etc.). And some Events use their own type of Energy.

If you have no Energy, then you can wait until it refills over time, or buy some for Diamonds. Or join the Royal Club and forget about Energy limitation for good!