If you are a new customer to BT you become eligible for the gift after 14 days have passed but it can take up to 28 days after that and possibly longer with the current crisis, for the email to be sent to you so you can claim your Xbox from the BT Shop, who will then dispatch it.
Unfortunately its not going to be possible at this time, for the forum moderators, who are the only BT employees on this forum, to investigate these promotions, as they are dealing with priority welfare escalations only.
You can try ringing BT Billing on 0800 800 150
Can you tell me how to solve this then please
Apart from ringing BT, there is not a lot you can do.
If its the offer shown on this page, make sure you were actually eligible.