It clearly has very serious corruption but provided that's only in the visible file system (region 48) and not in some other essential region like 41 you should be able to recover it. Try again with RMPrepUSB and if it still fails to format it in FAT32 try with Low Level Format Tool:
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Perhaps even MiniTool or EaseUS free partition managers can handle it. Then afterwards you can do the format again properly with RMPrepUSB so that it's acceptable for Garmin's OS. There's more than one way to skin a cat (or format in FAT). Garmin devices formatted in windows are often problematic and if done with a mac inevitably give trouble.