Can you use a reaction at any point on your turn or can a reaction be only played in response to your opponent playing an action?

Yes, you can play Reaction cards on your turn whenever you have priority — they don't need something to respond to.

Reaction grants all the permissions of Action (Rule 813.1.b), meaning it can be played during Open States on your turn like any regular spell. The Reaction keyword additionally lets you play it during Closed States (when a chain exists) on any player's turn (Rule 813.1.c.1).

So on your turn, you can:

• Play a Reaction to start a chain with an empty stack, just like an Action spell.
• Play additional Reactions in response to something already on the chain.
• Play a Reaction during a Showdown Open State (when you have Focus/Priority).

The only restriction is that during your opponent's turn, you generally need a trigger (a spell, ability, or Showdown) to gain priority and play a Reaction — you can't just fire one off randomly. But on your own turn, treat them as fully functional spells you can play at any appropriate timing window.