Paypal smart button server integration in PHP: Error: Expected an order id to be passed

I'm trying to implement a server side integration for smart button paypal. I've tried to fix this for days now. I'm new at this, it's a personal project, I'm trying to learn.

I keep getting 'Error: Expected an order id to be passed' every time that a click on the paypal button. What am I missing?

This is my client-side code:

paypal.Buttons({
// Customize button (optional)
style: {
    color: 'blue',
    shape: 'pill',
},
//Set up a payment
// This function sets up the details of the transaction, including the amount and line item details.
createOrder: function(){
    return fetch('<?php echo URLROOT; ?>/libraries/Paypal.php', {
        method: 'post',
        headers: {
        'content-type': 'application/json',
        },
    }).then(function(res){
        //console.log(res.json());
        return res.text();
    }).then(function(data){
        return data.orderID; // Use the same key name for order ID on the client and server
    });
},
// Execute the payment
onApprove: function (data) {
    //This function captures the funds from the transaction.
    return fetch('<?php echo URLROOT; ?>/libraries/Paypal.php', {
        method: 'post',
        headers: {
            'content-type': 'application/json'
        },
        body: JSON.stringify({
            orderID: data.orderID
        })

    }).then(function (res) {
        return res.json();
    }).then(function(orderData){
        if (errorDetail && errorDetail.issue === 'INSTRUMENT_DECLINED') {
          return actions.restart();
        }
        if (errorDetail) {
            var msg = 'Sorry, your transaction could not be processed.';
            if (errorDetail.description) msg += '\n\n' + errorDetail.description;
            if (orderData.debug_id) msg += ' (' + orderData.debug_id + ')';
            return alert(msg);
        }
      alert('Transaction completed by ' + orderData.payer.name.given_name);
  });
},
onError: function (err){
    alert('An Error occured ' + err);
}
      
}).render('#payment-btn');

This is my server-side:

// Creating an environment
$clientId = "HIDDEN";
$clientSecret = "HIDDEN";
$environment = new SandboxEnvironment($clientId, $clientSecret);
$client = new PayPalHttpClient($environment);
// Construct a request object and set desired parameters
// Here, OrdersCreateRequest() creates a POST request to /v2/checkout/orders
use PayPalCheckoutSdk\Orders\OrdersCreateRequest;
$request = new OrdersCreateRequest();
$request->prefer('return=representation');
$request->body = array(
    'intent' => 'CAPTURE',
    'application_context' =>
        array(
            'locale' => 'en-US',
            'return_url' => URLROOT.'/bookings/payment',
            'cancel_url' => URLROOT.'/bookings/payment'
        ),
    'purchase_units' =>
        array(
            0 =>
                array(
                    'amount' =>
                        array(
                            'currency_code' => 'EUR',
                            'value' => $_SESSION["total-price"]
                        )
                )
        )
);
try {
    // Call API with your client and get a response for your call
    $response = $client->execute($request);

    return $response->result;

    // If call returns body in response, you can get the deserialized version from the result attribute of the response
    print_r($response);
}catch (HttpException $ex) {
    echo $ex->statusCode;
    print_r($ex->getMessage());
}
use PayPalCheckoutSdk\Orders\OrdersCaptureRequest;
// Here, OrdersCaptureRequest() creates a POST request to /v2/checkout/orders
// $response->result->id gives the orderId of the order created above
//$orderId = $response->result->id;
$request = new OrdersCaptureRequest($orderId);
$request->prefer('return=representation');
try {
    // Call API with your client and get a response for your call
    $response = $client->execute($request);

    // If call returns body in response, you can get the deserialized version from the result attribute of the response
    print_r($response);
}catch (HttpException $ex) {
    echo $ex->statusCode;
    print_r($ex->getMessage());
}