Hi, @nit01 Welcome to the community! Thanks for reaching out.
Were you able to get this worked out? If not, I have a few things to help the community better understand your issue.
Questions:
To confirm, you created your Private App successfully? And were able to include the scopes mentioned in the article?
If not, where exactly in the process are you getting the error? When creating the Private App or when creating the Custom Object?
Additionally, can you paste your request, request body, and the response here? Along with the error body? You can use the code block formatter to help things from getting unwieldy.