The computer you’re trying to connect to may not be available because:
The computer is in sleep mode, is turned off or not plugged into a power outlet
See this article to learn more.
The network or Internet connection is not available
Make sure that your Mac or iOS device is connected to a Wi-Fi network and has access to the Internet if you're trying to connect to a remote computer.
The computer is on a different network
If you are connecting remotely to a Mac or Windows PC, make sure that Screens Connect is installed on the remote computer and that you are signed in with your Screens ID in the Screens app.
See these articles for more information regarding Screens Connect installation instructions: