I've took the exam before and it's listed in the Transcript section but if I click on it I'd only view the result from the last try (2017) but no option to buy a new voucher.
The ordering of the voucher is harder then the test ?
I've also wrote @learn but no solution after 2 days - looks like Extreme doesn't want my $$$.
Did you take the exam before? Please take a look at 'Certification' element either in that list or in your 'Transcript', from there you should be able to reach the exam if it is enabled for you.