Paypal api – PayPal payouts sandbox is giving AUTHORIZATION_ERROR

I am using PayPal payouts sandbox API. In my sandbox app, payouts are enabled. But when I hit the API endpoint then it gives AUTHORIZATION_ERROR although my client_id & client_secret are correct. Following is the code sample:

function get_paypal_auth_token($client_id, $client_secret) {
    $curl = curl_init();

    curl_setopt($curl, CURLOPT_URL, "https://api.sandbox.paypal.com/v1/oauth2/token");
    curl_setopt($curl, CURLOPT_HEADER, false);
    curl_setopt($curl, CURLOPT_POST, true);
    curl_setopt($curl, CURLOPT_RETURNTRANSFER, true); 
    curl_setopt($curl, CURLOPT_USERPWD, $client_id.":".$client_secret);
    curl_setopt($curl, CURLOPT_POSTFIELDS, "grant_type=client_credentials");
    $result = curl_exec($curl);
    curl_close($curl);
    return json_decode($result);
}

function create_paypal_payment($data) {
    $client_id = "XXXXXX";
    $client_secret = "XXXXXXXSECRET";
    $auth_data = get_paypal_auth_token($client_id, $client_secret);
    $url = "https://api.sandbox.paypal.com/v1/payments/payouts";
    $data_json = json_encode($data);
    $curl = curl_init();
    curl_setopt($curl, CURLOPT_HTTPHEADER, array('Content-Type: application/json', 'Authorization: Bearer '.$auth_data->access_token));
    curl_setopt($curl, CURLOPT_POST, 1);
    curl_setopt($curl, CURLOPT_URL, $url);
    curl_setopt($curl, CURLOPT_POSTFIELDS, $data_json);
    curl_setopt($curl, CURLOPT_RETURNTRANSFER, true);
    $resp = curl_exec($curl);
    curl_close($curl);
    return json_decode($resp);
}

The expected result of this API should be kind of related to payouts.

Also, I try it with another account’s credentials then it gives INSUFFICIENT_FUNDS error.

So can anyone please tell how to add balance to my PayPal account?