Why isn't this Ground Check system working?

I followed a tutorial by Code Monkey on YouTube on how to make a ground check system but for some reason it always returns null. Tutorial: https://www.youtube.com/watch?v=c3iEl5AwUF8

And here's the ground check system:

     private bool IsGrounded() // Check if the player is grounded
     {
         float extraHeightText = 0.01f;
         RaycastHit2D raycastHit = Physics2D.Raycast(collider.bounds.center, Vector2.down, collider.bounds.extents.y + extraHeightText, platformLayerMask); // Do a raycast to see if the player is  on the ground
         Color rayColor;
         if (raycastHit.collider != null)
         {
             rayColor = Color.green;
         }
         else
         {
             rayColor = Color.red;
         }
         Debug.DrawRay(collider.bounds.center, Vector2.down * (collider.bounds.extents.y + extraHeightText), rayColor);
         Debug.Log(raycastHit.collider);
         return raycastHit.collider != null;
     }