The location of the error log file in cPanel depends on the type of error you are looking for. Here are some common locations:
General cPanel errors:
/usr/local/cpanel/logs/error_log 12
/usr/local/cpanel/logs/error_log
Apache errors:
/usr/local/apache/logs/error_log 1
/usr/local/apache/logs/error_log
PHP errors: /usr/local/php/logs/error_log 3
/usr/local/php/logs/error_log
FTP errors:
/var/log/messages 4
/var/log/messages