Your seller may have created the tracking number for your item immediately after your order was placed, but the item might not have been dispatched to the Royal Mail collection center yet.
It is recommended that the tracking details be checked again within 48 hours. The status will be updated once Royal Mail has received the item at their collection center.
If the issue persists after one day, you should contact the sender to verify that the correct tracking number was provided and to confirm whether the package has been dispatched.