I have the same problem as you. I submitted a ticket to Databricks "Help to re-schedule assessment day in webassessor", but they responsed as below:

" Please accept my apologies for the inconvenience caused and the delay in responding.

I'm sorry to inform you that we will be unable to reschedule the exam after November because the voucher will expire on November 30, 2022.

Even, I regret to inform you that we do not have the option to extend or change the validity of the voucher once it has been released. "

This case maybe help other people to pick a day in webassessor.

Thanks all