Diagnose Javascript error

Hello

I’m doing some seo on my site and it’s been flagged that there is a .js error somewhere on the site.

I’ve tried to use this https://codex.www.ads-software.com/Using_Your_Browser_to_Diagnose_JavaScript_Errors
to help diagnose it but it say to put define('SCRIPT_DEBUG', true); before “That’s all, stop editing! Happy blogging”.I can’t see “That’s all, stop editing! Happy blogging” on the wp-config file though.

Where should I add the code?

Thanks