Thanks for reaching out. It looks like your TV isn't compatible with the Voice Remote. Please take note that although there are TV brands that can be set up for Chromecast with Google TV however, not pairable/compatible with the Voice Remote.
As a workaround, you may try to choose a different brand from the list, which in your view has the same country of manufacture as the TV. You may also search for TV remote codes online and see if any other brand matches the same code as Medion, and select that other brand instead. However, please be informed that this step may or may not work.
Thanks for the help here, davidking.