You can not mix using ‘babylonjs’ and ‘@babylonjs/…’ the packages are incompatible.
Either you use together the bundled version:
import * as BABYLON from ‘babylonjs’;
import “babylonjs/loaders”;
or you only use the scoped packages
import * as MYBABY from ‘@babylonjs/core’;
import “@babylonjs/loaders”;
This is a dupp of Skybox removed after loading model in v4.x but not in v5 alpha - #9 by Darcey