I'm not an expert, and I'm not CERTAIN I've been where you've been, but I
think I'm a little further along.
In general, this error means that the redirect URL you are specifying in the request doesn't match the ones configured on your account (from which the token comes).
Check to make sure the redirect URL in the request matches one of the Authorized Redirect URIs in your project's configuration:
Authorized URIs are listed at the bottom. You should? have been here previously. You can add them as needed.