Thank you for the additional information. KMLs can only be imported into an existing map as they contain features (points, lines, areas), and as such are not the map file itself. This is why you are seeing that error message. To import this file:
1. Open a map that covers the area you need
2. Tap the map features icon (pin with 3 lines) at the bottom right of the screen)
3. Depending on which platform you are using tap the ‘+’ icon (Android) or the square with the downward arrow (iOS) to import features
4. Navigate to where the KML is stored and select it for import
Please let me know if this works!