If you are accessing the course offer portal https://offer.ndors.org.uk and your the driving licence number you have entered is not recognised you should follow the instructions as published on that screen.
This is because the driving licence you use to access course offers has to match exactly the driving licence number you gave to the police when responding to their correspondence. If it doesn’t it will be rejected by the offer portal.
Police force contacts can be found by clicking this link: https://www.police.uk/forces/
If not, consider contacting us.