Can I cast Tactical Retreat on a unit in base to save it from a kill spell or damage?

Yes, you can cast Tactical Retreat on a unit that is already in your base to "save" it from a kill spell or damage.

Why this works:

1. Targeting: Tactical Retreat requires you to "Choose a friendly unit." It does not contain any restriction stating the unit must be at a battlefield. Therefore, it is a legal target whether the unit is on a battlefield or in your base (as confirmed by the principles in RiftJudge FAQ #9929).
2. Replacement Effect: When Tactical Retreat resolves, it creates a replacement effect for that unit: "The next time it would die this turn, heal it, exhaust it, and recall it instead."
3. Interaction with Kills/Damage: If a kill spell or lethal damage would cause that unit in your base to die later in the turn, the replacement effect will intercede. Instead of the unit being sent to the trash, it will be "healed, exhausted, and recalled" (sent to your base). Since it is already in your base, the "recall" effect simply confirms its current location, and it will effectively "survive" the event that would have killed it.

Important Note:

While you can protect a unit in your base this way, remember that Tactical Retreat only creates a "shield" for the next time that specific unit would die this turn. Once it has been triggered and the unit is healed/recalled, the effect is consumed.