s390x images (which installation artefacts) are not described correctly.

https://docs.openshift.com/container-platform/4.15/installing/installing_with_agent_based_installer/preparing-to-install-with-agent-based-installer.html

For KVM on s390x ISO is the recommended way to install. For zVM on s390x iPXE installation method is the way to go.

The following Note (Currently ISO boot is not supported on IBM Z....)