Reply To: 404.php problem after editing

Escape single quotes (‘) by using a backslash (\)

Example:

$text = 'Here's an example';

$text = 'Here\'s an example';

You can get the unmodified file here:
https://themes.svn.wordpress.org/frontier/1.2.4/404.php

Changing Text on a WordPress theme:
https://ronangelo.com/change-or-translate-text-on-a-wordpress-theme/