Ant is checking the database connection and if the postgresql extension pgcrypto is set up. The error message you posted shows that ant tries to connect to postgres on the computer you run ant (as localhost) and on port 5432. But, there is no database responding. Please check your database settings in dspace/config/local.cfg or if that does not exist in dspace/config/dspace.cfg. If you want to use a database on the same machine on which you are running ant, make sure it is properly installed, running and listening on localhost:5432.