Can I react to the Navori Fighting Pit trigger even if the units holding are already buffed?

Yes, you can react to the Navori Fighting Pit "Hold" trigger, even if the units at the battlefield already have a buff.

Why you can react:

According to the Showdown Flow and the Chain Rules, triggered abilities (like "When you hold...") are placed on the chain. Placing an item on the chain creates a "Closed State," which allows both players to play [Reaction] cards.

Even if the units holding the battlefield are already buffed, the trigger condition ("When you hold here...") has been met. The ability is placed on the chain and becomes a valid target for reactions.

Important Note on Targeting and Resolution:

While you can react to the trigger, keep in mind how the buffing mechanic and timing work in Riftbound:

1. Buff Mechanic (Rule 426.1.b.1): A unit can only have one buff counter at a time. If you target a unit that is already buffed, the effect will not grant it another buff.
2. Target Selection (FAQ #9501 / #7655): Triggered abilities like the one on Navori Fighting Pit select their targets when the trigger finalizes on the chain (before anyone has a chance to react). If your goal is to react to the trigger to give a specific hidden unit a buff, you cannot do so, because the target for the Navori Fighting Pit trigger will have already been locked in before you gain priority to play your reaction.

Summary: You can play [Reaction] cards in response to the trigger being on the chain, but because the ability targets during the finalization process, you cannot change the target of the Navori Fighting Pit's buff once it has been placed on the chain.