What's telling you what's corrupted?
Is it the Garmin, or the computer, and is it saying the card's corrupted or the gmapsupp.img file?
Also, how big is the card? (i.e. GB, or SD, SDHC, or SDXC)
SDXC cards (bigger than 32GB) are formatted exFAT by default, and many things won't read them because the makers have to pay Microsoft a royalty to use the exFAT filesystem. Windows won't format a card bigger than 32GB as FAT32, but other devices & software will.
Assuming that the gmapsupp.img file is one you've downloaded, and that the card is otherwise new, I'd try formatting it in the Garmin (which will be slow), then trying again. As noted earlier, the gmapsupp.img file has to be in a "Garmin" folder, otherwise it won't be found.
More caution will be required if it's a paid for card with OS data on it, or similar.