Got the following errors trying to install aptitude on a clean Ubuntu 18.04 on DigitalOcean:
$ sudo apt install aptitude
The following packages have unmet dependencies:
aptitude : Depends: libapt-pkg6.0 (>= 1.9.11~) but it is not installable
Depends: libboost-iostreams1.71.0 but it is not installable
Depends: libcwidget4 (>= 0.5.18-1) but it is not going to be installed
Depends: libgcc-s1 (>= 3.0) but it is not installable
Depends: libncursesw6 (>= 6.1+20180210) but it is not installable
Depends: libstdc++6 (>= 9) but 8.4.0-1ubuntu1~18.04 is to be installed
Depends: libtinfo6 (>= 6.1+20180210) but it is not installable
Depends: libxapian30 (>= 1.4.14~) but it is not going to be installed
Recommends: libparse-debianchangelog-perl but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
There was a file pointing apt to focal instead of bionic. Find it, edit it and just change to bionic:
$ grep focal -r /etc/apt/
/etc/apt/sources.list.d/archive_ubuntu_com_ubuntu.list:deb http://archive.ubuntu.com/ubuntu/ focal universe