I'm sorry to hear that you did not receive your voucher. I have reached out on your behalf to that team (with your email looped in) and also forwarded your request to someone who has helped with this issue in the past. In order to resolve this, you would normally need to reach out to this email: challengesupport@microsoft.com
You can also leave a comment on this Tech Community page: https://techcommunity.microsoft.com/t5/microsoft-learn-blog/boost-your-security-skills-with-the-new-30-days-to-learn-it/ba-p/3729953
The Microsoft Cloud Skills Challenge program is supported by a separate team that does not monitor this forum, but we should be able to resolve this using the options provided. If they are slower to respond though, I would recommend reaching out on Tech Community.
If the information helped you, please Accept the answer. This will help us as well as others in the community who may be researching similar issues.