This looks like a PHP 8 error, not a PHP 7.4 error.
see:
https://forum.osticket.com/d/98675-php-8-0-compatibility-error
Are you running 7.4 or 8?
You can create a test file (let's call it test.php) and put it in your webserver root folder.
In the file put <?php phpinfo(); ?>
browse to the file with your web browser. It should tell you what version of PHP you are running, and configuration information.
edit: once your done with the test.php make sure that you delete it.