You need to run psql as the user postgres, so do:
sudo su postgres -
Then run the postgres client:
psql
To list the databases, run:
\l
(Thats a lowercase L by the way!)
You should get something like this:
postgres=# \l
List of databases
Name | Owner | Encoding | Locale Provider | Collate | Ctype | Locale | ICU Rules | Access privileges
-----------±---------±---------±----------------±------------±------------±-------±----------±----------------------
gvmd | _gvm | UTF8 | libc | en_US.UTF-8 | en_US.UTF-8 | | |
postgres | postgres | UTF8 | libc | en_US.UTF-8 | en_US.UTF-8 | | |
template0 | postgres | UTF8 | libc | en_US.UTF-8 | en_US.UTF-8 | | | =c/postgres +
| | | | | | | | postgres=CTc/postgres
template1 | postgres | UTF8 | libc | en_US.UTF-8 | en_US.UTF-8 | | | =c/postgres +
| | | | | | | | postgres=CTc/postgres
Then run your ALTER command.