it seems that it's been too long, and you are still struggling about the issue. if you really tried your best and try to find solution every day and not find it.
curl -u CLIENT_ID:CLIENT_SECRET -d "grant_type=client_credentials&scope=premier" -X POST
https://oauth.fatsecret.com/connect/tokenfor that command example even though I don't use curl I try that command in cmd terminal, it is very straightforward and work perfectly just make sure your client is correct. please check again your client id and secret in your account. Good morning