Repository does not have a release file (18.10)

You have a mix of issues here.

FIRSTLY, you have a lot of references still to quantal. Quantal has been End of Life for ages, and is no longer supported. You will need to find where those lines are in your /etc/apt/sources.list file and within /etc/apt/sources.list.d/*.list files and remove those entries.

Secondly, you have some PPAs referring to Cosmic. Those PPAs are likely not supporting Cosmic and therefore is why you get the 404s there. remove those entries as well.

The repositories are still available on http://old-releases.ubuntu.com/ubuntu. You can change them in /etc/apt/sources.list, replacing all occurences of http://archive.ubuntu.com with http://old-releases.ubuntu.com.

After that, apt-get upgrade will work again.

More info on dealing with no longer supported versions at https://help.ubuntu.com/community/EOLUpgrades