不能在函数内声明private作用域。
在Start()函数中，删除private关键字。将此行更改为...

private GameObject objectToSpawn = squareslist[index];

...到这个...

GameObject objectToSpawn = squareslist[index];

MS文档...
private (C# Reference)

除了@quaabaam给出的答案之外，您不能在类级别的作用域中使用var关键字。
因此，要修复这两个错误：

var squareslist = new List<GameObject>{greenSquare, blueSquare, pinkSquare, greySquare};
    
void Start()
{
    for (int i = 0; i < 5; i++)
    {
        int index = random.Next(squareslist.Count);
        private GameObject objectToSpawn = squareslist[index];
            
        Vector3 newPos = new Vector3(objectToSpawn.transform.position.x + moveX, objectToSpawn.transform.position.y, objectToSpawn.transform.position.z);
        Instantiate(objectToSpawn, newPos, objectToSpawn.transform.rotation);
        moveX = moveX + moveXIncrease;
    }
}

应为：

private List<GameObject> squareslist;
    
void Start()
{
    squareslist = new List<GameObject>{greenSquare, blueSquare, pinkSquare, greySquare};
    for (int i = 0; i < 5; i++)
    {
        var index = random.Next(squareslist.Count);
        var objectToSpawn = squareslist[index]; 
        var newPos = new Vector3(objectToSpawn.transform.position.x + moveX, objectToSpawn.transform.position.y, objectToSpawn.transform.position.z);
        Instantiate(objectToSpawn, newPos, objectToSpawn.transform.rotation);
        moveX += moveXIncrease;
    }
}

现在有另一个问题。预置不存储位置。你试图从预置列表中读取位置不会产生你想要的结果。如果你想设置"产卵点"，你可能想添加空的游戏对象到你的场景，并使用那些位置为你的新示例化的游戏对象。但这是另一个问题，与最初的问题无关。