Keith,
You seem to be missing nothing. Tested this in postman with my own account and API key and then this works correctly.
I get the same error as you when part of the credentials is incorrect, so either the user name or API key seems to be invalid.
Maybe an additional space character somewhere?
Hi there, just for anyone stumbling over this post and still getting authorization errors: API Key of an App is NOT an API Key of the user profile. Would have helped me so maybe now helps you ;-)