Exam certification voucher don't work from Microsoft email

I completed Microsoft training and received a 50% discount voucher, but it isn't working when I try to use it. I've already tried using the correct Microsoft account, clearing my cache/using Incognito mode, and testing it on multiple devices. I also attempt for submitting a ticket through the Engage Center https://engagecenter.microsoft.com/#home, but the page is stuck loading, and tried to call a customer support but no one has been answered https://support.microsoft.com/en-us/support/. Can someone please assist me with this?