The certificate will be recieved on your registered email address so plz check using all email tab as there is a possibility it may have moved in spam somehow. Also, the same can be checked on this link too: https://academyportalcloud.mendixcloud.com/index.html
I faced the same issue in this month, I did wait for week after giving exam and wrote an email to academy <academy@Mendix.com>.
That helped me to get certificate.
Hope this helps!
This issue happened for mostly associate gave exam on year end, wait for couple of day then you can send mail to <academy@Mendix.com>.