See the ppc64 build here: https://copr.fedorainfracloud.org/coprs/praiskup/module-postgresql-11/build/836464/
That ppc64le chroot launched key generatoin but backend timed out while waiting for the key. When other chroots were launched, the key was already generated so they succeeded. It's the problem with key generation on copr-keygen that takes too long (4 to 5 minutes currently).
keygen problems were fixed by: https://pagure.io/copr/copr/pull-request/730