Ok, here's my thinking:
It does not say "you may" or "up to 2" it says "attach 2 basic energy". Therefore, if you cannot attach 2 basic energy you cannot meet the requirements of the attack and hence can't do it.
This would also make it easier to enforce: If you attach 2 energy it works, it not, it doesn't!
(I know with fisherman we were allowed to just take as many as we had if not 4 but remember that the discard is public knowledge. the hand isn't, hence the difference)
This is just a wrong interpretation, All that is necessary for an attack is that you satisfy the energy requirement. Everything else is just a situation where you do as much as you can. All comparisons to trainer or supporters may not be valid. For example, it is legal to play an attack for no effect. It is not legal to play a trainer or supporter for no effect.