Typically users are unable to attach an email to NetSuite because the role they are signing in with does not have the appropriate permissions for the specific record type or the roles does not have web services permissions.
For permission related issues contact your NetSuite Administrator and ask them to add permission for the record you are trying to attach to
To ensure your role has access to web services please share this article with your NetSuite Admin
Check that all of your prerequisites have been met
If you're still having trouble contact us using the chat icon at bottom right or send an email to email@example.com. Please provide as much detail as possible. Screenshots are welcome!