Magento – Magento 2 Error: Class does not exist

chown -R www-data:www-data /var/www/magento2/ 
chmod 777 -R var
chmod 777 -R generated
chmod 777 -R app/etc
rm -rf var/cache/* var/page_cache/* var/generation/*

and

php bin/magento setup:di:compile;

You just redeclare Custommodule_ReviewRating in your path instead of just one,

   <?php 
    namespace Custommodule\ReviewRating\Helper;

    class Data extends \Magento\Framework\App\Helper\AbstractHelper { 

        /** * @var \Magento\Framework\App\Config\ScopeConfigInterfac 
            */ 
        protected $_scopeConfig; 
        CONST ENABLE = 'reviewrating/general/enable_module'; 


        public function __construct( \Magento\Framework\App\Helper\Context $contex

t, 
            \Magento\Framework\App\Config\ScopeConfigInterface $scopeConfig ) {

             parent::__construct($context); $this->_scopeConfig = $scopeConfig;
    }

    public function getEnable(){
        return $this->_scopeConfig->getValue(self::ENABLE);
    }


}