An account is not listed in the server,the site is in the server and the nslookup is in the server and i can have access in cpanel (domain.com/cpanel) ,it just doesn't appear in WHM>list accounts.
Sometimes when pushing security updates we need to expand the list of reserved usernames prohibited for account creation which may explain the issue you are describing. If the account name is listed in either /etc/aliases or /etc/localaliases, you can either remove the alias from /etc/aliases or /etc/localaliases, or rename the account using the WHM API to a username not listed in /etc/aliases or /etc/localaliases: WHM API 1 Functions - modifyacct - Software Development Kit - cPanel Documentation