Unsupported PHP Version
From Joomla! Documentation
Joomla 4 requires PHP 7.2.5 or higher. See the Technical Requirements page.
Updating Your PHP Version
There's a variety of ways to update your PHP version. The main ones are covered in an article in the September 2020 issue of Joomla Community Magazine: How Do I Update My PHP Version?.
If you're using cPanel or Plesk there should be an option to upgrade PHP in your dashboard. If there is no option or you are unsure about how to achieve this, contact your host. Using modern PHP Versions is essential for keeping your site secure. If your host doesn't offer a version above 7.2.5 consider alternative hosting!
If you're developing locally you need to download an updated version of the local hosting tool you are using e.g. XAMPP