The following information may help to resolve the situation:

The following packages have unmet dependencies:
 appstream : Depends: libappstream4 (>= 0.10.2) but it is not going 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