Make sure objects are not instantiated on each other

Instantiate your obstacle at a random point, then Instantiate the coin at a random point. Check if the coin intersects with the obstacle. If not then you're done, else you wanna pick a new random position and repeat the check until you find a position that's free.

Something like this in code:

 public GameObject coin = null;
 public GameObject obstacle = null;

 void Start()
 {
     GameObject obstacleInstance = null;

     // Decide if we want an obstacle
     if(Random.value < 0.5f){

         Vector3 position = Vector3.zero; // Add your logic tp Calculate a new position here
         obstacleInstance  = (GameObject)Instantiate(obstacle, position, Quaternion.identity); // Instantiate obstacle at calculated position
     }

     // Decide if we want a coin
     if(Random.value > 0.5f){

         // Create a coin at a random position
         Vector3 position = Vector3.zero; // Add your logic to Calculate a new position here
         GameObject coinInstance = (GameObject)Instantiate(coin, position, Quaternion.identity);

         if(obstacleInstance){
             
             // If there is an obstacle calculate a new position until the  objects don't intersect

             while (coinInstance.renderer.bounds.Intersects(obstacleInstance.renderer.bounds)) {
                 position = Vector3.zero; // Add your logic to Calculate a new position here
                 coinInstance.transform.position = position;
             }
         }
     }
 }