That plugin hasn't been updated since 2018 and was never tested beyond WordPress 5.x or PHP 7.x. If you're running PHP 8+ (which most hosts have moved to), old plugins like this will throw fatal errors.
If you still need RTA labeling, you can add the header manually in your theme's functions.php or via .htaccess without the plugin overhead.
Code:
<IfModule mod_headers.c>
Header set Rating "RTA-5042-1996-1400-1577-RTA"
</IfModule>
Or the meta tag:
Code:
<meta name="RATING" content="RTA-5042-1996-1400-1577-RTA" />