How to get error logs when I cannot login admin (admin cannot load, ends with 502, 504 error)?

Your webhost will know where they're located and how you can access them

I thought invision error logs, not server logs.

I debug log enabled

define( 'DEBUG_LOG', true );

so I need to know, where I can read errors, when I cannot log into admin.

DEBUG_LOG will enable debug logging, which isn't related to any error logging!

So where is debug log?

I feel you may be misunderstanding my colleague there. Debug logging is not what you want to achieve what you are looking to achieve.

If you cannot get to the ACP the first thing you want to do is to set recovery mode, as shown here

https://invisioncommunity.com/4guides/client-services/getting-support-r292/#recoverymode

You may also see errors in /uploads/logs on your file system