Exporting maps to GPS

Yes - it's not tremendously helpful when that happens is it? :)

If it's any consolation, that page is better than it used to be. Below the "export" button there's now some text that says:

If the above export fails, please consider using one of the sources listed below:

Overpass API
    Download this bounding box from a mirror of the OpenStreetMap database
Planet OSM
    Regularly-updated copies of the complete OpenStreetMap database
Geofabrik Downloads
    Regularly-updated extracts of continents, countries, and selected cities
Metro Extracts
    Extracts for major world cities and their surrounding areas
Other Sources
    Additional sources listed on the OpenStreetMap wiki

However, if you want files for a Garmin GPS, you actually don't want any of these - you want instead want one of these or these. There are lots of options, but if you want a pre-made map the easiest to use and most popular is probably http://garmin.openstreetmap.nl/.

If you have a non-Garmin GPS, you'll need to search the wiki or search and then ask here saying "I have XYZ GPS and I'd like OSM maps on it; what do I need to do?"