This is the worst thing about Garmin. There have been several occasions when I've mentioned (somewhat tersely) they should have a look at the way TomTom updates...
As you're running Edge I'm guessing you're running Windows 10. The last time I spoke to Garmin they said they do not support updating from Win10. Pretty poor from a company supposedly at the cutting edge of technology and usability. They told me to connect using a Win 7 computer!
Try using Garmin Connect, or whatever the app is called. I managed to update the maps on my Nuvi with it, although there were some unlock issues. The latest version of BaseCamp overcomes the unlock issues. Unlock issues onlyn occur if you use the Garmin with a PC.
HTH
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