This error occurs when the machine you are trying to connect to is offline, or the link you are using has expired.
Check that the machine is online, ask for a new link if applicable, and restart Parsec on both ends if necessary.
Note
If you're logged in to Parsec using a public email service - Gmail, Yahoo, Hotmail, etc - and attempting to connect to a computer that is authenticated using a business domain this means your company uses Parsec for Teams and you'll need to reach out to your IT staff to be added to the Team.