Does it show up in the Extensions list for your Raspberry Pi still? I’m thinking perhaps the issue could be that it’s already been added and you’re trying to put another one on top of it.
If it does, we’re missing an option to remove extensions in the Cayenne UI at this time, but you can remove it manually if you’re comfortable editing JSON by removing it from the /etc/webiopi/devices.json file. You’d be looking for an entry similar to this:
(yours will vary a little bit from what I just posted there, so if you’re not 100% sure, feel free to post the output from cat /etc/webiopi/devices.json and I’ll highlight what to delete.)
If you’re interested in editing that file, you could alternatively remove and re-add the entire Raspberry Pi entry as this will give you a fresh slate.