This error happens when you've requested to connect to a computer but the request wasn't accepted. When someone requests to join, the host should see a notification and a section in the interface. They can accept the connection via the interface or by pressing CTRL+F1.
Windows notifications might not show up if it's disabled or in Focus Assist mode. In that case, just accept via Parsec's interface.
I'm joining my own machine / I can connect without approval
The machine may have lost internet access, crashed, or turned off unexpectedly. Because the computer disappeared unexpectedly, it will still show up in the computers list for a while. If you were connected before this happened, you will also see an icon showing that someone is connected; that someone is you from before the computer disappeared.
If possible, go to the physical location where the computer is or ask someone to check that it is working correctly, and that Parsec is running and the internet is working. If you're using a cloud PC, use the cloud PC provider's control panel to restart the machine.