Can I target only units in base with Piercing Light?

No, you cannot target only a unit in your base with Piercing Light.

According to the card text and RiftJudge FAQ #8905, Piercing Light requires you to "Deal 2 to a unit at a battlefield" as the first part of its effect. Because this is a mandatory action required to initiate the spell's resolution, you must have a valid target at a battlefield to play the card.

Additionally, as per RiftJudge FAQ #8481, Piercing Light must target two different units if you choose to use both parts of the effect. The first target must be a unit at a battlefield, and the second target must be a different unit (which can be at a battlefield or in a base). You cannot play the spell if there is no valid target at a battlefield.