Are you actively using the lineone email address?
Using the lineone email address and its password if you try the sign in to TalkTalk Mail (click here) Tell us if there's an error message or if you can sign in ok.
Are you a TalkTalk broadband customer? If so you don't need to sign up to Mail Plus just add the lineone mailbox to your MyAccount.
Please do a quick check that your Community Profile includes in Personal Information (Click here) your name, current landline 'phone number and an alternate number (mobile recommended) and address with postcode (location). Then scroll down to Private notes to add email address(es) with any relevant other notes and finally save changes. TalkTalk's Community Team OCE will then locate your registered TalkTalk details.