CS1529#

A using clause must precede all other elements defined in the namespace except extern alias declarations

This error occurs when using statements are not at the top of a script.

Resolution#

Move any using statements to the top of the script.

public class Example : MonoBehaviour
{
    ...
}

//  Incorrect
using UnityEngine;

//  Correct
using UnityEngine;

public class Example : MonoBehaviour
{
    ...
}

If you have put multiple MonoBehaviours or ScriptableObjects in one file you may have script loading issues. Separate the classes into different files to fix that issue.