Hosting can get blocked if port 5309 is not open on your local network. You can check if it's open or if something else is using the port following the tips here. Please open port 5309 if it's closed via Windows Firewall. Here's how you can do that.
Additionally, you must be on Windows 8.1, Windows 10, or their equivalent Windows Server versions to host. Windows 7 and 8.0 are not capable of hosting with Parsec.