psql 9.5: gen_random_uuid() not working
you can check if the functions are defined using

select pg_get_functiondef(to_regproc('gen_random_bytes'));
select pg_get_functiondef(to_regproc('gen_random_uuid'));


or:

select * from pg_proc where proname like 'gen_random_%';


if both functions are not defined then you probably had an error with the extension creation - just drop it and recreate:

drop extension pgcrypto;
CREATE EXTENSION pgcrypto;