Lux, Crownguard

No. Lux's ability can add energy to pay a spell's costs while that spell is being played, but it cannot supply the energy demanded by Hard Bargain.

Paying a spell's costs is part of the process of playing it. During that step, its controller can activate Reaction abilities that add resources to pay those costs.^{[357.1][357.1.a]} Once the spell is finalized on the chain, its costs have already been paid and opponents receive an opportunity to play reactions.^{[359.3.a][359.3.c]}

Hard Bargain's instruction, "Counter a spell unless its controller pays 2," is executed later, when Hard Bargain resolves.^[158.1] That payment is not a cost of playing the targeted spell. Although abilities that add resources can normally be activated whenever a spell or ability requires resources to be paid, Lux restricts her added energy to playing spells.^[429.3] It therefore cannot be used for Hard Bargain's payment or for another payment required while a spell or ability resolves.