how to make the player can use an ability just after a specific time

In whatever script you have your ability using in add this:

 float countDown;
 float time = //however many seconds;
 
 bool canUseAbility;
 //if this bool is true, you can use the ability
 
 void Start{
 canUseAbility = false;
 countDown = time;
 }
 
 void Update{
 if(countDown <= 0){
 canUseAbility = true;
 }
 else{
 countDown -= Time.deltaTime;
 //count down
 }
 }
 //as an example, to use the ability you will press space
 if(Input.GetKeyDown(KeyCode.Space))
 {
 if(canUseAbility){
 //Use ability
 canUseAbility = false;
 countDown = time;
 }