Upgrade ubuntu 18.10(comic) to 20.04(focal)
Ubuntu 18.10 has been end of life since 18th July 2019. It looks like they have now removed it (cosmic) completely from the apt repository.
You can still get Ubuntu 18.10 packages if you change your apt sources (/etc/apt/sources.list) to point to http://old-releases.ubuntu.com/ubuntu/. Note that these are not maintained and no security fixes will be applied.
A much better alternative, if you can, is to upgrade your system to a later version (eg: Ubuntu 20.04 LTS). There's an official tutorial on how to upgrade from 18.04 to 20.04.
The technique used should also apply to upgrading from 18.10, however please note that you may not be able to upgrade directly from 18.10 to 20.04. This may require multiple "hops"; meaning upgrading via another version or two to get all the way to 20.04. Follow below procedure to upgrade unsupported (18.10) version to latest
mkdir /tmp/comicupgrade
cd /tmp/comicupgrade
wget http://old-releases.ubuntu.com/ubuntu/dists/disco-updates/main/dist-upgrader-all/current/disco.tar.gz
tar -xf disco.tar.gz
python3 ./dist-upgrade.py
Do not turn off or reboot system between upgrade.
Once you are successfully updated to 19.04(Disco) you need upgrade to 19.10(eoan), again follow the same procedure.
mkdir /tmp/eoanupgrade
cd /tmp/eoanupgrade
wget http://old-releases.ubuntu.com/ubuntu/dists/eoan/main/dist-upgrader-all/current/eoan.tar.gz
tar -xf eoan.tar.gz
python3 ./dist-upgrade.py
