I have the same issue. I can't book the theory test. I even registered my European driving license with the d9 counterpart. But the number provided is not working. I'm trying since month to solve it. I contacted the DVLA, the DVSA and Pearson. It is hard to get through and none of the above could help me so far. They keep sending me back and force between each other. I'm not sure if I will ever find a solution.