Ubuntu 18.04 (bionic) reached end of life in June 2023.
We don’t publish images for distributions that are end of life.
(Yes, I’m aware that Canonical provides extended support past the regular end of life, but you can’t build your own images with the extended support packages as that requires a subscription)
You can definitely import those 18.04 images in Incus.
If there are no active repositories, you can set up http://old-releases.ubuntu.com/ubuntu/. Obviously, there will be no updated packages or security updates, but who I am to judge if you want to just set up a honeypot. Note that you can create a launchpad dot net account, and get for free the extended support for up to three systems.