BUG #15593: Unable to install PostgreSQL

The following bug has been logged on the website:

Bug reference:      15593
Logged by:          AnonymousID AnonymousID
Email address:      anonymous_id57@foxmail.com
PostgreSQL version: 11.1
Operating system:   RedHat6
Description:

URL:https://www.postgresql.org/download/linux/redhat/

yum install
https://download.postgresql.org/pub/repos/yum/11/redhat/rhel-6-x86_64/pgdg-redhat11-11-2.noarch.rpm

prompt
[root@localhost ~]# wget
https://download.postgresql.org/pub/repos/yum/11/redhat/rhel-6-x86_64/pgdg-centos11-11-2.noarch.rpm
--2018-01-19 02:03:44--
https://download.postgresql.org/pub/repos/yum/11/redhat/rhel-6-x86_64/pgdg-centos11-11-2.noarch.rpm
Resolving download.postgresql.org... 87.238.57.227, 72.32.157.246,
204.145.124.244, ...
Connecting to download.postgresql.org|87.238.57.227|:443... connected.
ERROR: cannot verify download.postgresql.org’s certificate, issued by
“/C=US/O=Let's Encrypt/CN=Let's Encrypt Authority X3”:
  Issued certificate not yet valid.
To connect to download.postgresql.org insecurely, use
‘--no-check-certificate’.