Have you followed the instructions in this post?
Specifically:-
Workaround
You can disable the message and the corresponding check by deleting these lines from your admin/index.php file. Note that the code, not the line numbers, should be checked, because the line numbers are different in different versions of phpList:
if (defined('RELEASEDATE') && ((time() - RELEASEDATE) / 31536000) > 2) {
Fatal_Error(s('Your phpList version is older than two years. Please %supgrade phpList</a> before continuing.</br>
Visit <a href="https://www.phplist.org/users/" title="'.s('Get some help').'">the support site</a> if you need some help.'
,'<a href="https://www.phplist.com/download?utm_source=pl'.VERSION.'&utm_medium=outdated-download-forced&utm_campaign=phpList" title="'.s('Download the latest version').'" target="_blank">'));
return;
}
Fix