name of a ppa for ppa-purge

You need to look into the list files. It many distributions it is found under /etc/apt/sources.list.d/.

For instance, I have a java ppa. The list is webupd8team-java-trusty.list.

It contains:

deb http://ppa.launchpad.net/webupd8team/java/ubuntu trusty main
# deb-src http://ppa.launchpad.net/webupd8team/java/ubuntu trusty main

So the ppa-purge command will look this way:

sudo ppa-purge ppa:webupd8team/java

For your "lubuntu" ppa the command is:

sudo ppa-purge ppa:lubuntu-dev/lubuntu-daily

The <ppa-name> is after http://ppa.launchpad.net/ and before the distro name.

If you still get errors, then your list is damaged. You can always add a PPA then purge it, e.g.:

sudo add-apt-repository ppa:lubuntu-dev/lubuntu-daily
sudo apt-get update
sudo ppa-purge ppa:lubuntu-dev/lubuntu-daily