SELECT gen_random_uuid()
produces output
ERROR: function gen_random_uuid() does not exist
SQL state: 42883
Hint: No function matches the given name and argument types. You might need to add explicit type casts.
I ran CREATE EXTENSION pgcrypto; on selected database and SELECT gen_random_bytes(1) works perfectly (gen_random_bytes doesn't work on other databases where pgcrypto extension was not manually created).
% psql --version
psql (PostgreSQL) 9.5.3
Ubuntu version is 16.04.