psql: error: connection to server on socket "/var/run/postgresql/.s.PGSQL.5432" failed: FATAL: Peer authentication failed for user "postgres" (Ubuntu)

When I try to open psql with this command:

psql -U postgres

I get this error:

psql: error: connection to server on socket "/var/run/postgresql/.s.PGSQL.5432" failed: FATAL:  Peer authentication failed for user "postgres"

But it connects successfully when I use:

sudo -u postgres psql

Can someone please explain what is happening and how to diagnose/fix this problem? My pg_hba.conf contains the following:

# Database administrative login by Unix domain socket
local   all             postgres                                peer

# TYPE  DATABASE        USER            ADDRESS                 METHOD

# "local" is for Unix domain socket connections only
local   all             all                                     md5
# IPv4 local connections:
host    all             all             127.0.0.1/32            scram-sha-256
# IPv6 local connections:
host    all             all             ::1/128                 scram-sha-256
# Allow replication connections from localhost, by a user with the
# replication privilege.
local   replication     all                                     peer

You can edit your .conf files with privileges using an editor, for my case it is nano.

$sudo nano /etc/postgresql/14/main/pg_ident.conf

Map your user by adding this line

# MAPNAME       SYSTEM-USERNAME         PG-USERNAME
user1           <computer-username>     postgres

Replace the <computer-username> with the System-Username, which can be found using the whoami command. Type in your terminal:

$whoami

Then go ahead to the pg_hba.conf with privileges

$sudo nano /etc/postgresql/14/main/pg_hba.conf

Add your postgres user, with method=peer, as shown below:

# TYPE  DATABASE        USER            ADDRESS                 METHOD
local   all             postgres                                peer

This worked for me.