Does ambush require Conquere Battlefield?

No, Ambush does not require you to conquer a battlefield.

According to Rule 822.1.b, Ambush is a passive ability that allows you to play a unit to any battlefield where you already control units. It does not require that you be in the process of conquering that battlefield, nor does it require you to conquer it after playing the unit.

Key Points:

• Valid Location: Ambush simply expands the valid locations where you can play a unit. As long as you have at least one unit already present at a battlefield, that battlefield becomes a valid location to play a unit with Ambush (Rule 822.1.c).
• Reaction Timing: Ambush grants the unit the [Reaction] keyword while being played to such a location, allowing you to play it even if the game state is Closed (such as during a Showdown or Combat).
• No Conquer Requirement: You can use Ambush to join an ongoing combat, to reinforce a battlefield you already hold, or simply to place a unit at a location where you already have a presence. It is not tied to the act of conquering.

This is confirmed by RiftJudge FAQ #8743, which clarifies that you do not need to control the battlefield itself to use Ambush; you only need to control units at that location.