You can enable php-fpm at domain level or webserver level. I will tell you about domain level
1) Go to Webservers -> Domain conf
2) Select the username for domain where you want to activate php-fpm
3) If there is no domain configuration, click the option to create a new configuration
4) In the configuration, select apache->php-fpm. If you are using nginx select nginx -> apache-> php-fpm
5) Select the version of php-fpm that you want
6) Rebuild and save the configuration
You should not get 503 error now on this domain.