Can I install php8.0 on Ubutnu 16.04
Solution 1:
They did explicitly warn you in the output you posted above:
Only Supported Versions of PHP (http://php.net/supported-versions.php) for Supported Ubuntu Releases (https://wiki.ubuntu.com/Releases) are provided. Don't ask for end-of-life PHP versions or Ubuntu release, they won't be provided.
You should not be running 16.04 as it has passed end of life. Upgrade first to a currently supported Ubuntu release, and then you will be able to install PHP 8.0.