This is an interesting new one. Currently Spacewalk 1.10 does not install on RHEL8/CentOS8 due to missing or updated packages (Java or Python2). These instructions contain the steps to install an untested Spacewalk Nightly server, adapted for Python 3.
Let's start off with a clean CentOS 8 minimal install.
dnf -y copr enable sbluhm/java-packages
dnf -y copr enable sbluhm/spacewalk-rhel8 # as soon as the official nightly builds, I will update this section
dnf -y config-manager --enable PowerTools
dnf -y module enable javapackages-tools:201801/common
dnf -y install epel-release langpacks-en langpacks-de glibc-all-langpacks
dnf -y install spacewalk-setup-postgresql #crypto-utils
dnf -y install spacewalk-postgresql
firewall-cmd --add-service=http
firewall-cmd --add-service=https
firewall-cmd --add-port=5222/tcp
firewall-cmd --add-port=5269/tcp
firewall-cmd --add-port=69/tcp
firewall-cmd --runtime-to-perm
spacewalk-setup