If you are trying to connect to a Mac, confirm that Remote Login is enabled and properly configured. If you are trying to connect to a Windows PC, install and configure a SSH server, such as freeSSHd, to provide the required secure shell (SSH) service. For Linux PCs, make sure OpenSSH is installed and configured properly.
Try signing in with Screens Connect. 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.
Keep in mind that it’s also possible that the remote computer might actually be unavailable due to any number of reasons, including a power or network outage or excessive network activity; try connecting again later.