Can you try again? I recently did an update to my maps and Express errored out a few times before it finally did the update.
I applied 3 actions and managed to solve the problem, after all Garmin recommendations and support pages on the internet proved useless. So I recommend the following, I'm not sure which one of the 3 really solved the problem:
1) format SD card and use 16 kilobytes allocation size for FAT32 (i used a windows 10 machine and 16GB microSD card)
2) stop all your downloads before starting the update
3) stop your antivirus
It worked for me.