May I use Ambipom [G]'s "Tail Code" attack if my opponent only has an Active Pokemon and no Benched Pokemon?
Yes you can, although it won't move or discard the energy. There must be a Pokemon on the bench or you can't move the energy off the defending Pokemon.
Source: PUI Rules Team (2010-01-28)

Relevant cards

Note: All applicable cards may not be listed below.
HGSS2_43
PL1_48
PL2_56