We have noticed that the repodata subdirectory at
https://download.postgresql.org/pub/repos/yum/9.6/fedora/fedora-26-x86_64/repodata
is empty and so yum/dnf commands fail with a message like:
Failed to synchronize cache for repo 'pgdg96', disabling.
This is only for this specific combination of Fedora and PostgreSQL
release versions 26-9.6. Other combinations have valid repodata,
e.g. 27-9.6 and 26-10.
Karl