Sites using PHP-FPM are stored in a different location than sites not using PHP-FPM. This article cover's how to find a PHP-FPM site's error log.
The log will be located in the following location. USER should be replaced with the cPanel account username for the account that owns the domain. EAMPLE_COM should be replaced with the site in question. Note that the underscore character is used in place of the period character.