You probably are using an older version of zp-config.php which has defined DEBUG. It is not really a problem. You can delete line 32 of functions.php or find the define in zp-config.php and delete it.
Or you can change the line in functions.php to `if (!defined('DEBUG')) { define('DEBUG', false); }`
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Or you can change the line in functions.php to `if (!defined('DEBUG')) { define('DEBUG', false); }`