Can I play Plumeria and discard two cards if my if my opponent has no energy attached to any of their Pokemon?
No, you cannot. Discarding the two cards is a cost not the effect, and you cannot play Trainer cards for no effect.
Source: TPCi Rules Team (2019-03-21)

Relevant cards

Note: All applicable cards may not be listed below.
SM3_120
SM3_145