I have solved this problem myself. The issue appears to be max distance/max altitude settings. If they are too low, Active Track will not engage, and the "Can't use focus track due to geo zone" error will appear, even if you are NOT in a geo zone. In my case, initial testing was done with both settings at 20 meters, and increasing them substantially resulted in focus/active track engaging properly without any error message. I haven't tested extensively enough to know what the exact threshold settings are for this to work, or if both max distance and max altitude are involved -- it may be max distance only, but I'm not sure. I have recommended to DJI that the error message be corrected to note the distance setting issue and not erroneously claim that a geo zone is involved, and that the documentation/manual be updated to note this issue.