EpicCan I use Lux, Crownguard's ability to pay for Hard Bargain?
No. You can use Lux's ability to add energy to pay the costs of playing a spell, but Hard Bargain's payment is not a spell play cost.
Paying a spell's costs is part of finalizing that spell.357.1 During that payment, its controller can activate a [Reaction] ability that adds resources.357.1.a
Once the spell finishes finalizing, opponents receive the opportunity to play Reactions such as Hard Bargain.359.3.a359.3.c Hard Bargain's instruction, "Counter a spell unless its controller pays 2," is executed later, when it resolves.158.1 The payment is therefore not part of playing the targeted spell. You may still activate Lux's ability,430.3 but the energy it adds can be spent only to play spells and cannot be spent on Hard Bargain's payment.