PARTIAL TARGETING (NOT ALL UNITS MAY BE SELECTED OR AREN’T PRESENT)

If an effect requires targeting all units within a certain range (e.g., Reek's ability, "target all enemies in short range"), and one of the units cannot be targeted because of an effect that prevents the unit being targeted, can the ability still be resolved in this situation?

Yes, units that cannot be targeted are skipped when resolving abilities that affect multiple units (in the mentioned example, all units except the one with the target blocking ability would receive the Panicked token).

Follow up: If I play a tactics card like Unbowed Unbent Unbroken while I control the sword zone, but there is no enemy in long range, what happens?

You resolve only the part of the card that is valid, (in this case the panic test reroll/auto pass.)