If you're trying to drag and drop a card on your Waffle board and see a "Could not move card #123" error, this is usually due to Waffle not being approved as a 3rd party application.
You can go to here to see if Waffle is listed: https://github.com/settings/connections/applications/b68f04bdff231220a275
If Waffle is approved you will see this page and there should be a green checkmark next to the org Waffle is approved for:
If you don't see Waffle listed there, you will you'll need to log out of Waffle and log back in to be prompted to authorize Waffle and create webhooks. (this only applies if you're an owner in your org) If you're not an owner, you'll want to have one do the steps above.