Check out example codes for "destroying assets is not permitted to avoid data loss". It will help you in understanding the concepts better.
Code Example 1
This is because the variable obstacle is referencing the asset. What you need to do, is change Instantiate(obstacle, new Vector3(x * 7.5f, y, z),Quaternion.identity); to obstacle = (GameObject) Instantiate(obstacle, new Vector3(x * 7.5f, y, z),Quaternion.identity); Instead of destroying the instantiated object you were trying to destroy, you were actually attempting to destroy the original due to not assigning obstacle as the instantiated object.
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