First... check that you have the lines permissioning to the myuser user in pg_hba.conf. For example:

# IPv4 local connections:
host    all             all             127.0.0.1/32            md5
# IPv6 local connections:
host    all             all             ::1/128                 md5

Or any other lines of permission to IPV4 (and IPv6 if you use) with: TYPE DATABASE USER ADDRESS METHOD

After this check, run the psql as follows:

psql -h localhost -U myuser mydatabase

And then, the requested prompt, enter the user's password myuser.

First of all, check if you have the lines giving permission to the myuser user in pg_hba.conf. For example:

# IPv4 local connections:
host    all             all             127.0.0.1/32            md5
# IPv6 local connections:
host    all             all             ::1/128                 md5

Or any other lines of permission to IPV4 (and IPv6 if you use) with: TYPE DATABASE USER ADDRESS METHOD

After this check, run the psql as follows:

psql -h localhost -U myuser mydatabase

And then, the requested prompt, enter the user's password myuser.