Following are the common reasons:

• There could be a typo in the address set as reset/notification address. Please click on I haven’t received the email -> Edit email address.
• The email address you provided to receive Mailfence account activation code might have put that email in the Spam folder or refused it. Please make sure to check the Spam folder OR include ‘noreply@mailfence.com’ in respective account’s whitelist (then follow the next point).

You can request another activation code by clicking on Send new code.

If the account registration fails, please check this KB article.