GameObject is already being activated or desactivated

Questions & Answers
1

When I jump into the moving platform they show my this error.

void OnCollisionEnter2D(Collision2D other)
    {
        if (other.transform.tag == "MovingPlatform")
        {
            transform.parent = other.transform;
        }
    }

    void OnCollisionExit2D(Collision2D other)
    {
        if (other.transform.tag == "MovingPlatform")
        {
            transform.parent = null;
        }
    }

The error is this line of code:

transform.parent = null;
2

On possible cause: This error is shown if you are trying to set the parent more than once in the same frame. So the Enter and Exit might be happening in the same frame (for some reason).

3

Several things you could do first is remove the OnCollisionEnter and have a debug log on your OnCollisionExit to check if it really go through. Debug.Log is a really powerful to help you figure out what is really going on.

From what you told, the gameobject doesnt have a parent to begin with. It might be because two functions are firing at the same time.