This might happen for a few reasons, the most common one is that you aren't able to reach the Windscribe API to process the login. If this is the case, try restarting the app or even the Firestick device and then try the login again.
The network itself might also be interfering with the API connection so if possible, connect the Firestick to a different WiFi network or try using a hotspot from your phone connected to mobile data. Using a different network can help get around those restrictions. You only need to log in once as after that, the server list will be available and connecting to servers doesn't require access to our API.
You might also have the incorrect date and/or time set on your device. Make sure that the date and time on your device is set correctly as it is necessary for the connection to our servers.