PHP 7.4 Warning: Use of undefined constant und - assumed 'und' (this will throw an Error in a future version of PHP) in eval()

The error says it's happening in code running through the "eval()" function, that means someone added custom code to the "PHP contextual filter code" field on a default argument on a view. Please take a look at the page where this error is displayed, examine the view(s) on that page and review the code in the page's arguments.

There are several reasons for this like:
nvalid character in a variable name, Using a class constant without stating the scope of the class, Using a constant that does not exist in this PHP version I find this article discussing it well.