https://www.postgresql.org/download/ I click CentOS beside 'Red Hat,'
enter the following;
Platform: RedHat Enterprise, CentOS, Scientific or Oracle version 7
It works for me, so I suspect there may be a network connectivity
problem (firewall?). Maybe you should try downloading the RPM in some
other way and see if that works. If it does, you can install it by
running 'rpm -ivh' on the downloaded file.