How to Install PostgreSQL 12 on CentOS 8 RHEL 8
use yum or dnf
yum install install https://download.postgresql.org/pub/repos/yum/reporpms/EL-8-x86_64/pgdg-redhat-repo-latest.noarch.rpm
yum update

yum list postgresql-server
yum install postgresql-server

yum module disable postgresql
yum list postgresql12-server
yum install postgresql12-server

postgresql-setup –initdb
or
/usr/pgsql-12/bin/postgresql-12-setup initdb

systemctl enable postgresql
systemctl start postgresql
systemctl status postgresql

su postgres
psql