Maybe a stupid question: did the navigation worked with the original SD-card?
I now see there is also a Golf version "from 2015". Does the Discover unit looks more like that picture?
The difference there is that if you download it, you get a folder "maps".
The Golf "until 2015" is for a Discover Media generation 1 and the "from 2015" is for a Discover Media generation 2 (newer model).
Me myself I got a VW Passat Variant 2016, so I choose the "from 2014" version.
That is a Discover Media generation 2 model and that works well.
There is an other way to try.
VolksWagen got its own download manager.
If you have choose your car and device you see on top of the maps the Discovercare update manager.
Download that and install it on you computer.
Set back the original backup at your SD-card and start the discovercare manager.
It will help you to download and install the new map on your SD-card.
Warning: the download with Discovercare takes a very long time.
Peter