When Lucian Gunslinger attacks and there is only 1 deflect unit there, can he pass his ability or does he have to do it?

He can't skip it outright, but he can get out of it by declining the Deflect cost.

Lucian's "When I attack" trigger is mandatory (no "may"), so it goes on the Chain as a Pending Item. If the Deflect unit is the only enemy unit there, he must choose it as the target (Rule 402.4) — that's not optional. However, Deflect imposes a Mandatory Additional Cost, and at the Pay Costs step he may decline to pay it (Rule 404.2). If he declines, the trigger is removed from the Chain and never resolves — no damage.

Net result: he can effectively "pass" on the damage, but the trigger still appears and the Deflect unit must be chosen first. (Rule 404.2)