I've escalated a request for you to get authorisation for removal of the unused email addresses so that you'll have space to add and manage up to a total of 5 active mailboxes in your MyAccount including your tiscali mailbox.
The likely reason you're not able to remove unwanted email addresses from MyAccount is because they're not showing on the active database and therefore MyAccount doesn't find them for the Delete process to work.
If you have a spare space in the 5 allowed in MyAccount but you're finding that the tiscali mailbox, although active, isn't adding to your MyAccount it's probably because that tiscali email address is already linked to a former MyAccount. It'll need detaching from that former MyAccount before you'll be able to add it to one of the 5 slots for mailbox management in MyAccount.
Please do a quick check that your Community Profile includes in Personal Information (Click here) your name, current TalkTalk landline 'phone number and an alternate number (mobile recommended) and address with postcode (location).
Then scroll down to Private notes to clearly note the email address(es) for removal and mailbox address(es) you want to add to MyAccount with any relevant other notes and finally save changes.
TalkTalk's Community Team OCE will then locate your registered TalkTalk details and authorise removal of the unwanted addresses.
I've alerted OCE_Ady that you've received a mail message about the tiscali email being restricted to webmail only. This means you'll be able to access the mailbox via the TalkTalk Mail sign in.
Did the unwanted mailboxes get removed from your MyAccount to allow you to add the wanted tiscali mailbox? If the tiscali mailbox is not yet added to your MyAccount then that's the reason you got the email notification.
Managing your email in My Account has advice and the section to look at is Email not showing in My Account.