Are you having the same issue as me? https://forums.garmin.com/showthread.php?89319-Message-when-sending-route-to-Montana
I'm not sure what your particular issue is but I run 3 nuvis and an Etrex 20 off Basecamp without problems, and have done so for years. It recognises all the maps on all the devices, this includes Garmin CN maps and numerous OSM maps, and I can transfer routes/waypoints between them all. That doesn't help your particular problem I know so hopefully the Basecamp team can resolve it for you, as there must be something strange about your particular setup/device.
Perhaps you could confirm what OS you're running and whether the map is on a card or the device, and how you obtained it?