What happens if I use Morpeko V's "Electro Wheel" attack but I don't have any Benched Pokemon?
You still have to discard an Energy if you can, but since there are no Benched Pokemon to switch with, Morpeko V stays in the Active Spot.
Source: Sword & Shield FAQ (2020-01-24), TPCi Rules Team (2020-02-06)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH1_190
SWSH1_190
SWSH1_79
SWSH1_79