Remove the entry of that park domain from these files and try to park the domain again.

/var/cpanel/users

/etc/httpd/conf/httpd.conf

/etc/localdomains

/etc/userdomains

/etc/valiases/

/etc/vdomainaliases

/var/named

/etc/named.conf

/etc/passwd

/etc/shadow

/etc/trueuserdomains

/etc/trueuserowners

/etc/vfilters

/etc/vftp

/etc/vmail