Ok this is a tricky one

Ok this is a tricky one. Base Clefairy's Metronome states:

Chose 1 of the Defending Pokemon's attacks. Metronome copies that attack except for its Energy costs and anything else required in order to use that attack, such as discarding Energy cards. (No matter what type the Defending Pokemon is, Clefairy's type is still Colorless.)

But Movie promo Moltress's hyper flame states:

Flip a coin. If heads, discard 1 Fire Energy card attached to Moltres. If tails, discard all Energy cards attached to Moltres. If you can't discard Energy cards, this attack does nothing.

If you can't discard Energy cards then that attack does nothing. So when Metronome is used on Moltres it doesn't work right?

Interesting question here. The answer is this: Discarding energy is an effect of the attack, not a cost of it. Thus, Clefable must discard the energy. Funny thing is this. Flipping tails will result in Clefable just ditching the 1 energy needed for Metronome. Flipping heads means the attack will fail unless the energy attached to Clefable is fire. So, tails is actually the better flip. Funny, eh?

Scott