Continuous Heart Rate Monitoring from Google Fit App

Question

I'm developing an app for Android (only for smartphone) and I want to monitor continuous heart rate using googleApiClient (Google Fit) from smartwatch (Polar Unite fitness band).

What I have done to find data source:

/** Finds available data sources and attempts to register on a specific [DataType].  */
private fun findDataSource(){
    Fitness.getSensorsClient(this, getGoogleAccount()).findDataSources(
        DataSourcesRequest.Builder()
            .setDataTypes(DataType.TYPE_HEART_RATE_BPM)
            .setDataSourceTypes(DataSource.TYPE_RAW)
            .build())
        .addOnSuccessListener { dataSources ->

            for (dataSource in dataSources) {
                Log.i(TAG,"Data source found: $dataSource")
                Log.i(TAG,"Data Source type: ${dataSource.dataType.name}")

                if (dataSource.dataType == DataType.TYPE_HEART_RATE_BPM && dataPointListener == null) {
                    Log.i(TAG,"Data source for TYPE_HEART_RATE_BPM found!")
                    registerFitnessDataListener(dataSource, dataSource.dataType)
                }
            }
            if (dataSources != null && dataSources.size == 0){
                Log.i(TAG,"No data source found")
            }
        }
        .addOnFailureListener { e -> Log.e(TAG,"failed",e) }
}

No Data Source found for heart rate but when I change DataType to DataType.TYPE_LOCATION_SAMPLE, it shows only my mobile phone as a data source and Missing polar watch. Can anybody let me know the reason or where I am making mistake?