If you see "connection unavailable" there are 2 possibilities:
1. Your Browser/Firewall/VPN is preventing access to sandbox.neo4j.com which is something you must address on your end. Can you access the sandbox via sandbox.neo4j.com?
2. The sandbox is no longer running because it expires after 3 days. Try clearing your Browser cookies and reload the GraphAcademy page. This will recreate the sandbox instance for this Challenge. It will take a minute or so for you to see the $neo4j prompt, meaning that you are connected.