Hello,
I just opened an issue over at the github repo. It looks like the links for postgres 12 rpms (and I just noticed the archive) are all returning 404.
github issue: https://github.com/pgdg-packaging/pgdg-rpms/issues/139
v12 rpms: https://download.postgresql.org/pub/repos/yum/12/
archive: https://download.postgresql.org/pub/repos/yum/repopackages#pgredhatoldrepos
Thanks for your assistance,
Lane Russell