The error that this solution fix:
Get:4 http://ppa.launchpad.net/ondrej/php/ubuntu focal InRelease
Err:4 http://ppa.launchpad.net/ondrej/php/ubuntu focal InRelease
Clearsigned file isn't valid, got 'NOSPLIT' (does the network require authentication?)
Go the to the source list folder:
cd /etc/apt/sources.list.d
Edit the file:
nano ondrej-ubuntu-php-focal.list
The file content should look like this:
deb http://ppa.launchpadcontent.net/ondrej/php/ubuntu focal main
# deb http://ppa.launchpad.net/ondrej/php/ubuntu focal main
# deb-src http://ppa.launchpad.net/ondrej/php/ubuntu focal main
Now you should be able to update as usual:
sudo apt update