Can you use Terrakion EX's "Pump-up Smash" attack if you don't have 2 basic Energy cards in your hand?
Yes, you can. You do the 80 damage then attach either one or no Basic Energy, depending on what you have in your hand.
Source: TPCi Rules Team (2012-08-23)

Relevant cards

Note: All applicable cards may not be listed below.
BW6_121
BW6_71