I have tried to upgrade the PHP version. but I just get errors.

I tried this tutorial.

apt update
apt upgrade

apt install software-properties-common
add-apt-repository ppa:ondrej/nginx

apt update
apt upgrade

apt install php8.0 php8.0-cli php8.0-fpm php8.0-apcu php8.0-opcache 
php8.0-common php8.0-pear php8.0-curl php8.0-zip php8.0-mcrypt php8.0-mysql 
php8.0-mbstring php8.0-xml php8.0-gd php8.0-gmagick php8.0-imagick 
php8.0-xmlrpc php7.3-maxminddb

but I get this error.

Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package php8.0
E: Couldn't find any package by glob 'php8.0'
E: Couldn't find any package by regex 'php8.0'
E: Unable to locate package php8.0-cli
E: Couldn't find any package by glob 'php8.0-cli'
E: Couldn't find any package by regex 'php8.0-cli'
E: Unable to locate package php8.0-fpm
E: Couldn't find any package by glob 'php8.0-fpm'
E: Couldn't find any package by regex 'php8.0-fpm'
E: Unable to locate package php8.0-apcu
E: Couldn't find any package by glob 'php8.0-apcu'
E: Couldn't find any package by regex 'php8.0-apcu'
E: Unable to locate package php8.0-opcache
...

How can I solve this problem?

I have tried my tutorial. but just get same error like this.

As stated in the ppa description:

Only Supported Versions of PHP (http://php.net/supported-versions.php) for Supported Ubuntu Releases (https://wiki.ubuntu.com/Releases) are provided.

Ubuntu 16.04 is no longer supported since April 2021, so I think there is also no longer a xenial branch of php 8.0. I'd suggest to upgrade Ubuntu to ver. 20.04 LTS in order to be able to use php 8.0