It is not your turn. What do you want to do?
Counterspell - level 3 spell, reaction
Allows you to cancel a spell completely.
Always works if the spell is level 3 or lower, need to roll above 10 + spell level if above the level you're casting counterspell with.
Cutting words - bardic inspiration die, reaction
Roll a bardic inspiration die (1d8) and subtract the result from an attack roll, ability check, or damage roll.
You can use this after the dm has announced the number the enemy has rolled, but before you know it's if a success/failure.
Absorb elements - level 1 spell, reaction (from Verdant Viola)
Grant yourself resistance against one of the following damage types until the start of your next turn.
The first time you attack with your rapier on your next turn, the target takes an extra 1d6 damage of the triggering type.
Opportunity attack - reaction
Attack an enemy moving out of range with your rapier.