E: The repository 'http://ppa.launchpad.net/webupd8team/java/ubuntu focal Release' does not have a Release file.

It looks like the PPA doesn't support Java version you want to install/update in your Ubuntu.

Read this note: https://launchpad.net/~webupd8team/+archive/ubuntu/java:

...
The PPA supports Ubuntu 18.10, 18.04, 16.04, 14.04 and 12.04.
...

Try to find a different way to install Java 8.

e.g.

OpenJDK:

sudo apt-get update
sudo apt install openjdk-8-jdk

AdoptOpenJDK:

1. Java 8:
   https://adoptopenjdk.net/installation.html?variant=openjdk8&jvmVariant=hotspot#x64_linux-jdk
2. or Java 11:
   https://adoptopenjdk.net/installation.html?variant=openjdk11&jvmVariant=hotspot#x64_linux-jdk

Amazon Java Corretto:

1. Java 8:
   https://docs.aws.amazon.com/corretto/latest/corretto-8-ug/generic-linux-install.html
   https://docs.aws.amazon.com/corretto/latest/corretto-8-ug/downloads-list.html
2. or Java 11:
   https://docs.aws.amazon.com/corretto/latest/corretto-8-ug/generic-linux-install.html
   https://docs.aws.amazon.com/corretto/latest/corretto-11-ug/downloads-list.html

Note: read this article to select the default Java version if you have installed multiple Java versions.