I am working on a smart home skill. To enable account linking, I have successfully make the authorization URL work, I also got POST request from Amazon to my token URI. My server also reply with a "token". However, in the Amazon alexa app, it shows " An error occurred while attempting to link Alex with Netro. Please try again later". But there is no details of the error, where is wrong? Is there any way to debug and know the details of errors? What's the requirement for the token URI? I just reply a json with "token" values. Any descriptions of the token URI requirements? Thanks.