Seems like truffle didn't find a server. Is testrpc running? If not, open a new terminal and run the command
testrpc
If testrpc is running and you still have the problem, make sure you have configured the truffle.js file right.
truffle.js file should look something like this:
module.exports = {
networks: {
development: {
host: 'localhost',
port: 8545,
network_id: '*' //* will match to any network id
}
}
};