ちなみに、公式のサンプルでは
https://docs.unity3d.com/ja/2018.4/ScriptReference/Resources.LoadAll.html
private Object[] textures;
private GameObject go;

void Start()
{
textures = Resources.LoadAll("Textures", typeof(Texture2D));

foreach (var t in textures)

object型で定義して読み込んでますね