Hi @Sakib_Khondaker . Sorry you’re running into that issue.
First, unlink your Sketchfab and Epic accounts (from your Sketchfab account settings).
Next, relink your Sketchfab and Epic accounts. Make sure that you’re linking the same Epic account that you use for Fab sign-in.
If you continue to run into issues, please reach out to the support team and they should be able to provide a bit better support than I can. 