usageEventlogs process

it is in files/usageStats/

Where should I add this error_log?Line182?

Yes, before this line ojs/ at ojs-stable-2_4_8 · pkp/ojs · GitHub

Between lines 181 and 182 in the 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.