Skill Drain VS Effect Veiler VS Elemental HERO Wildheart

If Skill Drain was first on the field before Elemental HERO Wildheart was summoned, and then Effect Veiler is about to be used on the newly summoned Elemental HERO Wildheart, it cannot be used.

If Elemental HERO Wildheart was on the field first and then Skill Drain got activated, you can use Effect Veiler on Elemental HERO Wildheart. When Effect Veiler stops applying its effect then Elemental HERO Wildheart will go back to being unaffected by trap cards, Skill Drain included.