making a delay on particle collider spawn?

Now i see what you are trying to do. A Particle Collisionmodul is not a Collider Component! So you are trying to access something, thats not attached to your scriptholder. Use the following code :

  ParticleSystem particlesystemVariable ;
 ParticleSystem.Particle[] particles = new ParticleSystem.Particle[particlesystemVariable .particleCount];
 
      void Start()
          {
     
              particlesystemVariable = gameObject.GetComponent<ParticleSystem> ();
              StartCoroutine(DelayCollider());
         
          }
         
          IEnumerator DelayCollider()
          {
 for (int i = 0; i < particlesystemVariable .particleCount;i++){
 particles [i].velocity = 20f;
 }
 
                 var collisionVariable = particlesystemVariable .collision;
                 collisionVariable .enabled = false;
     
              yield return new WaitForSeconds(delayTime);
     
             var collisionVariable = particlesystemVariable .collision;
             collisionVariable .enabled = true;
     
          }
         
          // Update is called once per frame
          void Update()
          {
              
          }