If I have Arachnoid Horror in base, can I play Ambush unit (Nidalee) as a reaction when enemy tries to conquer an open bf?

No, you cannot play Nidalee using the Ambush keyword in this situation.

According to Rule 822.1.b, the Ambush keyword allows you to play a unit as a [Reaction] only to a battlefield where you already control units.

In your scenario:

1. Arachnoid Horror is in your base, not at a battlefield.
2. Because you do not control any units at the battlefield the enemy is attempting to conquer, you do not meet the requirement for the Ambush keyword.

Since you do not have units at that location, you cannot use the [Reaction] timing granted by Ambush to play Nidalee there.