Troubleshooting Mixpanel API: Can't Download Data, Need Help

Hi Team, I am trying to download data from mixpanel via api. I am following https://developer.mixpanel.com/reference/raw-event-export. I created a service account for the same. But when i put in base 64 encoded username:secret of the service account, I get 400 error and I am unable to figure out what I am doing wrong. Has anyone attempted and successfully downloaded the data from https://data.mixpanel.com/api/2.0/export in the recent past? Thanks