In order to use any power, you need to actually fulfill the conditions to use it. For example:
~You cannot use Uxie's Set Up if you have 7 or more cards in your hand (as per errata)
~You cannot use Claydol's Cosmic Power if you have no cards in your hand
~You cannot use Unown G's GUARD if all your Pokemon in play have Pokemon Tools attached
etc.
And also, trying to announce a power when you cannot use it or if it is known that the power will do nothing at all can be considered unsportsmanlike conduct (which is a punishable offense in tournaments).