Hello ,
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.
Please let me know if you have any additional questions or concerns.
Best,
Raghib