It means the registration for your YouView Box has not been found against your account. Probably needs re-registering.
Post in a reply here, the last 4 digits of the Device Identifier, found the Settings (Click here).
Important: To identify your account, the OCE that will re-register your box for you needs your TalkTalk service landline number put into your Community Profile, Personal Information (Click here). Check it's ok or amend or add and then save changes.
I'll move this post from Welcome to Help with your Service Forum (TalkTalk TV).
And welcome to Community.
... thanks for providing the device identifier and updating the profile. A request has gone out to an OCE to check and if needed re-register the box for you. The OCE will come back to you here to confirm. Allow a day, subject to ongoing workload.
Box Identifier: 21ed
Can you check the phone number on your Community Profile is correct, I'm not showing any services with that number.
OCE's Hours of Operation - Mon to Fri 8am to 4pm. Urgent issues outside these times, please use Live Chat