it is in files/usageStats/
Where should I add this error_log?Line182?
Yes, before this line ojs/UsageStatsLoader.inc.php at ojs-stable-2_4_8 · pkp/ojs · GitHub
Between lines 181 and 182 in the UsageStatsLoader.inc.php file. It is in plugins/generic/usageStats/
But if you unsure on editing the core files this way, I would suggest you find someone who can help you.