Seeing "the default PC audio input device has changed to an invalid device" when opening SDR

It sounds like you may have PC selected for your mic input. As such, it is looking for a NON DAX input device (like a microphone) in order to grab audio from somewhere to squirt into the radio. Many PCs these days don't expose the input audio devices unless there is something plugged into them, so there may actually not be an active input device in the typical case. You can check this by right clicking on the Speaker icon in the system tray (lower right typically) for Win10/Win11 and exploring the related windows there.

If you're still stuck and can't make it work, please open a helpdesk ticket and we'll see what we can do to help get you going.