i have Garmin Express 5.1.0.0, just manually updated it.
I have lifetime maps on my Zumo 590.
I cannot see how to update the maps on my Mac, i DO remember having a menu option in the past - which seems to have gone now.
I have seen that this is an ongoing problem with Express, but most of the older threads are closed and are actually about much older versions.
Is this a Sierra issue - or a Mac Book Pro issue or a Garmin Express 5.1.0.0 issue - i can get my maps on the Zumo - but the USB is slow so is a real pain to use those maps on the Zumo when i have such fast SSD storage on my MBP.