unity create cube in script
void Start()
{
/*GameObject != gameObject*/
GameObject cubeObject = GameObject.CreatePrimitive(PrimitiveType.Cube);
/*if you need add position, scale and color to the cube*/
cubeObject.transform.localPosition = new Vector3(0, 1, 0);
cubeObject.transform.localScale = new Vector3(1, 1, 1);
cubeObject.GetComponent<MeshRenderer>().material.color = Color.red ;
}