Here's a couple of tips:
Check the browser console. Perhaps the URN or the access token is incorrect, which is why the model does not load.
The link you're working with is for an older version of Forge Viewer (v6). I'd suggest using version 7.*. Try using this basic app as a starting point: https://github.com/petrbroz/forge-basic-app, and replace this line of code:
const viewer = new Autodesk.Viewing.GuiViewer3D(document.getElementById('preview'));
with
const viewer = new Autodesk.Viewing.Viewer3D(document.getElementById('preview'));
With this I've been able to load a model with a headless viewer:
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