Encountering this same issue, has this been resolved?

The issue I was having with my client secret was that I was accessing it incorrectly with my curl command — I had to change single quotes (``) to double quotes("") to get my client secret script to execute within the curl command properly: curl -v POST "https://appleid.apple.com/auth/token" -H 'Content-Type: application/x-www-form-urlencoded' -d 'grant_type=client_credentials' -d 'scope=user.migration' -d 'client_id=com.jumbofungames.platMaker' -d "client_secret=$(node clientsecret_jwt2.js)"