Can not drop a PostgreSQL database using dropdb?
What you see there with psql \l only means that the Postgres role admin has been granted a couple of privileges by the Postgres role postgres - which is the default superuser and probably the actual owner of the database.

Quoting the manual:


  If a superuser chooses to issue a GRANT or REVOKE command, the command
  is performed as though it were issued by the owner of the affected
  object. In particular, privileges granted via such a command will
  appear to have been granted by the object owner.


The privileges granted are:

C -- CREATE
c -- CONNECT
T -- TEMPORARY


CONNECT and TEMPORARYon databases are also default default privileges for PUBLIC, but what we see here has been granted explicitly.

The privilege to drop the DB is not given, it cannot be. Only the owner of the database can drop it. (Or a superuser.) Hence the error message:


  ERROR:  must be owner of database


Typically, each shell user connects with a Postgres role of the same name. So, given you have the necessary sudo rights, this should do it:

sudo -u postgres dropdb my_database_name_here


Or switch to the  shell user postgres like demonstrated here.

But you may have to disconnect other users first. See:


Force drop db while others may be connected