1.By default, workflow includes users who do not have site access. Users without site access who attempt to complete the task assigned to them are directed to the request permission page.
2.The user account has been disabled in AD. Re-enable her domain account.
3.You need to enable the alert for the user to access the task list part.
4.By default, workflow can include the external user who does not have site access. Ensure this user is a domain user.