Can I rebuke a unit my opponent tries to move into an open battlefield?

Yes, you can play Rebuke to target a unit your opponent is moving into a battlefield, provided you have the opportunity to play it.

Timing and Legality

• Rebuke is an [Action] spell. According to Rule 732.1.c.1, the [Action] keyword allows a spell to be played during your turn or during a showdown on any player's turn.
• When a player moves a unit to a battlefield, if that move causes the battlefield to become contested, a showdown is opened (Rule 429.1).
• Because a showdown is now in progress, you have the permission to play [Action] spells like Rebuke, provided you have the opportunity to do so (i.e., you have focus and the chain is empty).

Important Considerations

• Chain State: If your opponent's move triggers other effects (such as "When I move" or "When I attack" triggered abilities), those effects will be placed on the chain first. While those items are on the chain, the state is "closed," and you can only play spells with the [Reaction] keyword (Rule 309.1.a). You must wait for the chain to resolve and the state to become "open" before you can play an [Action] spell like Rebuke.
• Focus: During a showdown, players alternate focus. You can only play an [Action] spell when you have focus and the chain is empty (FAQ 4805).
• Targeting: Rebuke targets a unit at a battlefield. If the unit is successfully moved to the battlefield and the move is complete, it is a legal target for Rebuke.

In summary, you can play Rebuke against a unit being moved into a battlefield, but you must ensure the game state allows for an [Action] spell to be played (i.e., it is your turn to act in the showdown and no other effects are currently on the chain).

Note: I could not fully verify this ruling against the retrieved rules.