Comment 12 for bug 1556540

sudo ppa-purge ppa:kubuntu-ppa/ppa-landing

Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 kde-config-screenlocker : Breaks: plasma-workspace (< 4:5.5~) but 4:5.4.3-0ubuntu1 is to be installed
 libkscreenlocker5 : Breaks: plasma-workspace (< 4:5.5~) but 4:5.4.3-0ubuntu1 is to be installed
E: Unable to correct problems, you have held broken packages.
/usr/sbin/ppa-purge: line 191: aptitude: command not found
Warning: Something went wrong, packages may not have been reverted

Trying another reboot....