Today I tried to connect pgAdmin to PostgreSQL but failed. I got fe_sendauth: no password supplied error.

After googling, I found this:

Basically, it is important to understand that for most Unix distributions, the default Postgres user neither requires nor uses a password for authentication. Instead, depending on how Postgres was originally installed and what version you are using, the default authentication method will either be ident or peer.

More: https://chartio.com/resources/tutorials/how-to-set-the-default-user-password-in-postgresql

So in the default setup, the default user for Postgres is postgres and requires no password. The solution to allow pgAdmin to connect to Postgres is to change the authentication in Postgres from md5 to trust.

First, open pg_hba.conf by running:

sudo nano /etc/postgresql/10/main/pg_hba.conf

And change the line:

# IPv4 local connections:
host    all             all             127.0.0.1/32            md5

to:

# IPv4 local connections:
host    all             all             127.0.0.1/32            trust

After that, don’t forget to restart the PostgreSQL server.