Hi Juan,
The reason this issue is occurring is due to your Epic Games account's email address being unverified. Sketchfab requires a verified email address in order to access their libraries. Please follow the steps in the How to verify the email address on your Epic Games account - Epic Accounts Support article to verify your email address. Once this is complete, you should be able to access the Sketchfab library in Twinmotion.
Best,
Raghib