Can you play Wallop at 0 energy?

Yes, you can play Wallop at 0 energy, provided you pay its optional additional cost.

According to the card's text, Wallop has a base energy cost of 2. However, it includes the following ability: "As you play this, you may spend a buff as an additional cost. If you do, ignore this spell's cost."

When you choose to pay this optional additional cost by spending a buff, the spell's energy cost is ignored (effectively reduced to 0), allowing you to play it even if you have no energy available.