I am having the same problem but I have already got five email addresses linked to the account so I believe that is why.
However I have been unable to delete an email address to make a space for a new one. I am very concerned that there is a deadline for this .
I have spoken to talktalk and used the online chat but the issue has not been solved. Please will someone help asap. I have noticed someone called fao OCE Arne has helped others.
I've requested help from the talktalk team to remove a redundant email address from your MyAccount to enable you to add the wanted mailbox to the other 4 in your MyAccount.
The likely reason for not being able to remove the unwanted email address is that it's not an active mailbox on the active database so MyAccount isn't able to complete the removal proccess to leave you a free slot to add the wanted mailbox. Do you think this is the reason?
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 add the email address(es) for removal with any relevant other notes and finally save changes. TalkTalk's Community Team OCE will then locate your registered TalkTalk details, authorise the removal and let you know when that's done.