Thank you for contacting the Microsoft Community.
Allow me to provide assistance in rescheduling your exam at a testing center using the same discount.
If you have used the Microsoft Virtual Training Day discount to schedule the exam, please know once cancelled the voucherless discount will be reinstated to your profile after 24 hours. You may try re-linking the event registered email address to your profile to obtain the offer again.
On the other hand, if this is a physical voucher (Alphanumeric code), I would need further details on what voucher you were referring to, so that we can properly assist you and have these details checked by our Support team.
To proceed, kindly provide us with a PSR (Problem Steps Recorder) from the time you are signing in to your Microsoft Certification profile until the page where the discount is missing.
You may send the PSR to the private message I initiated along with the requested information (Refer on the image below)

I look forward to your response. Please note that if no response is received after two business days, the case will be closed and locked. After the case is locked, we will no longer be able to respond, even through Private Messages. Thank you.