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). Instead of writing up, how to compile each missing package, I will give you instructions how to install the Nightly Spacewalk Server on CentOS 8. Let's start off with a clean CentOS 8 minimal install.
dnf -y copr enable @spacewalkproject/python-packages
dnf -y copr enable @spacewalkproject/java-packages
dnf -y copr enable @spacewalkproject/nightly
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 update python3-dmidecode
dnf -y install spacewalk-setup-postgresql
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