h3d.scene.LightSystem has no field ambientLight

[min-pascal]

In the docs' Lights section under Ambient Lighting it says you set the scene ambient light color like

s3d.lightSystem.ambientLight.set(0, 0, 0);

[rcstuber]

Only the (older) forward light system has an ambient property. So you need to downcast to that if you are using the forward renderer like shown in the World sample.
cast(s3d.lightSystem,h3d.scene.fwd.LightSystem).ambientLight.setColor(0x909090);

As for ambient light with the PBR renderer, I'm not quite sure. Maybe through the environment?