By wanting to reset a password by going through the link "Lost password" of the login page, it is, indeed, asked to enter the email of your Universign account.
If this email no longer exists and is no longer accessible, it means that your Universign account can no longer be used to make signatures, because on this email address you receive the invitations to sign and the documents you have signed. In this case, you must revoke your current account using this link https://app.universign.com/revocation/ and recreate a new account with the desired contact information.