Can I use Sceptile's "Nurture and Heal" ability to attach a 
Energy to a Pokemon that doesn't have any damage on it?
Yes, you can.
Nurture and Heal
Sceptile
- Type
- POKEMON
- HP
- 130
- Energy Type
- Stage
- STAGE2
- Retreat Cost
- Weakness
x2- Rarity
- RARE
- Illustrator
- Hajime Kusajima
- Set
- Primal Clash (PRC)
- Number
- 8/164
Abilities
Nurture and Heal
Once during your turn (before your attack), you may attach a Energy card from your hand to 1 of your Pokémon. If you do, heal 30 damage from that Pokémon.
Attacks
Jungle EdgeRulings (2)
If I use Sceptile's "Nurture and Heal" ability, is that an extra Energy card attachment or does it count as my one attachment for the turn?
It is an extra Energy attachment. You still get to attach one other Energy card as your normal attachment for the turn.
Nurture and Heal