This is because the only way you can afford the Energy cost of the Event is to Forget at least one Power Card. The game automatically calculates what the minimum requirements will be. Once you Forget the minimum number of required cards based on the calculation, the discard option will become available if possible.

In the example below, the required cost is 16 Energy. The Spirits can pay 6 Energy directly. In order to cover the remaining 10 Energy, they must Forget 2 Power Cards, and then either discard or Forget 1 more Power Card. (There are no Sun cards in the discard piles)