You are triggering setting the boolean to true whenever you press the K key. You have a condition that set's the Attack animation to start if the boolean is true meaning that it will keep starting every tick, and you are using on animation finished to reset the boolean to false, which is never triggered because there is constantly an attack animation playing.
If you don't want to be able to spam the attack faster than the animation you need to add the condition to your K button event: Is attack animation playing (inverted).
This will only fire the trigger if the attack animation is not already playing. Then you need to add a trigger once while true condition to the event where you set the attack animation to play.
If you do want to be able to spam faster than the animation leave your trigger animation as it is. Add the trigger once condition to the next event, and then change the condition to reset the boolean to false from animation finished, to on key K released.