psql: FATAL: password authentication failed for user "postgres"

I am running postgresql-9.6 on Rhel7. I know that the default/root user doesnt have a password, however i will like to use phpPgAdmin to do my work. the problem is I have edited my pg_hba.conf file to look as follow, (based on various documentation I have read so far:

local   all             all                          password
# IPv4 local connections:
host    all             all       127.0.0.1/32       password
# IPv6 local connections:
host    all             all             ::1/128      ident

The only problem is that after this configuration even the default user postgres has lost access completely. When I try to log in as the root user with psql and and enter my computer's root user's password this is what what happens:

-bash-4.2$ psql
Password:
psql: FATAL:  password authentication failed for user "postgres"

but when I change the above configuration on the pg_hba.conf file to:

peer
ident
ident

I can log in as root however phpPgAdmin won't let me in without a password.