The "Mission Not Valid" error is probably showing because you don't have a flight path defined yet. You must double tap on where you intend to take off to define the estimated home point. From there you should be able to save the basemap for offline use.
The idea behind the map caching that is that you save the basemap and estimated mission while you have internet. When you go in to the field you can recall the mission that was saved and execute it. Also, you can delete the recalled mission and create a new one that reuses the cached basemap.
The download error is because of lack of internet or a low quality internet connection. We would recommend caching the tiles while on a solid WiFi connection to minimize this error.