Cannot login in as root user after MySQL installation (RHEL)
In the security section of the documentation it states:


  If you do not know the initial random password, look in the server
  error log.


Try running the following command. It works for me on my 5.7 installation (which has the same instructions as the 8.0 version of the documentation):

sudo grep 'temporary password' /var/log/mysqld.log


It should tell you the initial password to use for root.