Why is there no shadow on the plane I created?

As shown in the playground below, the ground that comes with the model can cast shadows, but the plane I created does not show shadows. I am very confused about this.

Looking forward to your reply!

The intensities of your lights are too high, it makes the shadows disappear. If you lower them a bit:

You can improve the quality of the shadow by tweaking the light position, shadowMinZ and shadowMaxZ properties and by setting a filter method on the shadow generator: