Hello ,
Try closing and re-launching your Twinmotion to see if this lets you download the Sketchfab assets.
If you still can't download the assets and you are signed into your Epic Games account inside Twinmotion please try closing Twinmotion then logging out of the Epic Games Launcher and logging back in.
If that still doesn't work, please let me know. If you can also provide a screenshot of the error message you're getting, that would be very helpful.
Best,
Raghib