Can I use an action to an opponent reaction?

No, you cannot.

According to the Showdown timing flow and Riftbound FAQ #9306, an Action spell can only be played when the state is Open (the chain is empty).

When your opponent plays a Reaction, they are starting or adding to a Closed State (a chain). While that chain exists, you are restricted to only playing cards or abilities with the Reaction keyword. You must wait for the chain to fully resolve and the state to return to Open before you have the opportunity to play an Action spell.

Why this happens:

1. Action spells require an Open State: You cannot start a chain with an Action if one is already in progress.
2. Reaction spells create a Closed State: When a Reaction is added to the chain, it closes the state, meaning no Actions can be played until that chain is cleared.
3. LIFO Resolution: The chain will resolve in "Last-In, First-Out" order. Even if you have priority, you are restricted by the current "Closed" state until the chain is empty again.

Sequence:

• A chain is currently open/active (e.g., your opponent played a Reaction). The state is Closed.
• You cannot play an Action spell.
• You may only play a Reaction card to respond to the existing chain.
• Once the chain resolves completely and the state becomes Open again, the player with priority may then play an Action spell.