Error with Oauth 2.0 Client Credentials workflow

Once source of error is possible the getEpochTimeMinute. Maybe it's generating an incorrect value, either because the system clock is wrong, or maybe you're getting seconds instead of minutes or something like that. What I use is Math.floor(Date.now() / 60000);

Other potential source of error is making sure you're sending to correct hash algorithm name when you make the call ("SHA512").