Well done for securing yourself this opportunity. I am sorry to hear you are having difficulty booking onto an assessment day. Check out the forum thread link posted in the comment above, and it may help for you to make a call to the hiring manager at the branch you were accepted to.
Should you manage to book a place, don’t forget to dress smartly, behave modestly, answer honestly, and act friendly as part of the team. I wish you every success in securing the job for which you have applied!