@darrene Funny, but I had to change the config to remove the square brackets and replace them with curly brackets around the config parameters to get the config.js file to be accepted:
{
    module: 'MMM-fitbit',
    position: 'top_center',
    **config: {**
        credentials: {
            client_id: <client_id>,
            client_secret: <client_secret>,
        },
        resources: [
            'steps',
            'floors',
            'caloriesOut',
            'distance',
            'activeMinutes',
            'sleep',
            'heart'
        ],
        update_interval: 60
    **}**
},