S Mode only supports installing applications from the Microsoft Store. Currently, Slashtop Streamer is not available in the Microsoft Store.
"Windows 10/11 in S mode" only works with compatible apps from Microsoft Store.
If you try to use an app, not in Microsoft Store, such as the "Splashtop Streamer App," you'll get a reminder that the app isn't an option when using Windows 11 in S mode.
If you still want to download the Splashtop Streamer App that's not in Microsoft Store, you can switch out of S mode, but you won't be able to revert.
Please open a support ticket if you need further assistance.