Flutter Stripe PaymentIntent status : requires_payment_method

I would greatly appreciate your help because I am stuck with the following problem.
In Flutter, I use the Flutter_Stripe package to implement a payment module.

Hereunder you will find my Flutter function :

Future<void> initPayment({required String email, required int montant, required String nomOrga}) async{
    try{
      //1. Crée un payment intent sur le serveur
      final response = await http.post(
        Uri.parse("myFirebase URL"),
        body: {
          "email":email,
          "amount": (montant*100).toString(),
        },
      );
      print("email : $email - montant : $montant - organisation : $nomOrga");

      final jsonResponse = jsonDecode(response.body);
      print(jsonResponse);

      //2. Initialise le payment sheet
      await Stripe.instance.initPaymentSheet(paymentSheetParameters: SetupPaymentSheetParameters(
        paymentIntentClientSecret: jsonResponse["paymentIntent"],
        merchantDisplayName: nomOrga,
        customerId: jsonResponse["customer"],
        customerEphemeralKeySecret: jsonResponse["ephemeralKey"],
        customFlow: true,
      )
      );

      //3. Présenter le paiement
      await Stripe.instance.presentPaymentSheet();
      print("on a présenté la paymentsheet");


      //4. Attacher la méthode de paiement au PaymentIntent et confirmer le paiement
      /*final paymentIntentResult = await Stripe.instance.confirmPayment(
          paymentIntentClientSecret: jsonResponse["client_secret"],
      );*/
      print("on a fini confirmpayment");
      //print(paymentIntentResult.status);

      /*if(paymentIntentResult.status == PaymentIntentsStatus.Succeeded){
        //Paiement réussi
        quickConfirm("Paiement réussi", "Le paiement est réussi", context);
      }else{
        quickAlert("Oups", "Le paiement a échoué", context);
      }*/

    }
    catch(error){
      if(error is StripeException){
        quickAlert("Oups", "une erreur StripeException s'est produiten${error.error.message}", context);
      }else{
        quickAlert("Oups", "une erreur brol s'est produiten${error}", context);
      }
    }
  }

and hereunder the server function:

const functions = require("firebase-functions");
const stripe = require("stripe")("sk_test_...");
require("firebase-functions/logger/compat");

exports.stripePaymentIntentRequest = functions.https.onRequest(async (req, res) => {
    try{
        let customerId;

        //Get the customer who's email matches the one sent by the client
        const customerList = await stripe.customers.list({
            email: req.body.email,
            limit: 1,
        });
        console.log(customerList);

        //Check if customer exists, if not create a new customer
        if(customerList.data.lenght !== 0){
            customerId = customerList.data[0].id;
        } else {
            const customer = await stripe.customers.create({
                email: req.body.email,
            });
            customerId = customers.id;
        }

        //Creates a temporarysecret key linked with the customer
        const ephemeralKey = await stripe.ephemeralKeys.create(
            {customer: customerId},
            {apiVersion: '2020-08-27'}
        );

        //Creates a new payment intent with amount passed from the client
        const paymentIntent = await stripe.paymentIntents.create({
            amount: parseInt(req.body.amount),
            currency: 'eur',
            customer: customerId,
            automatic_payment_methods: {
                enabled: true,
            },
        });

        console.log('on arrive au send');
        console.log(res);
        res.send({
            paymentIntent: paymentIntent.client_secret,
            ephemeralKey: ephemeralKey.secret,
            customer: customerId,
            publishableKey:'pk_test...',
            success: true,
        });
        console.log('on a fini 200');

        } catch(error) {
            console.log('il y a eu une erreur :', error.message);
            res.status(404).send({
                success: false,
                error: error.message,
            });
        }
});

I have the following problems :

a) As is I don’t receive any error but the payment is always incomplete with the status "requires_payment_method". I use the 4242… test card (note that I live in Belgium and the test card is a US one, is it the cause of the problem?).Note tat the status of the PaymentIntent request is 200. Hereunder, the result of it:

payment_intent.created
Afficher les détails de l'événement
Données de l'événement
{
  "id": "pi_...",
  "object": "payment_intent",
  "last_payment_error": null,
  "livemode": false,
  "next_action": null,
  "status": "requires_payment_method",
  "amount": 100,
  "amount_capturable": 0,
  "amount_details": {
    "tip": {
    }
  },
  "amount_received": 0,
  "application": null,
  "application_fee_amount": null,
  "automatic_payment_methods": {
    "enabled": true
  },
  "canceled_at": null,
  "cancellation_reason": null,
  "capture_method": "automatic",
  "client_secret": "pi_...",
  "confirmation_method": "automatic",
  "created": 1677946647,
  "currency": "eur",
  "customer": "cus_...",
  "description": null,
  "invoice": null,
  "latest_charge": null,
  "metadata": {
  },
  "on_behalf_of": null,
  "payment_method": null,
  "payment_method_options": {
    "bancontact": {
      "preferred_language": "en"
    },
    "card": {
      "installments": null,
      "mandate_options": null,
      "network": null,
      "request_three_d_secure": "automatic"
    },
    "eps": {
    },
    "giropay": {
    },
    "ideal": {
    }
  },
  "payment_method_types": [
    "card",
    "bancontact",
    "eps",
    "giropay",
    "ideal"
  ],
  "processing": null,
  "receipt_email": null,
  "review": null,
  "setup_future_usage": null,
  "shipping": null,
  "source": null,
  "statement_descriptor": null,
  "statement_descriptor_suffix": null,
  "transfer_data": null,
  "transfer_group": null
}

b) In the Flutter code, point4 (confirmPayment) is commented in the code sample. If I left it uncommented I receive an error, something like "Null is not a subtype of String"

c) If the customer doesn’t already exist in Stripe I receive an error (it probably comes from the javascript code, do I have to test for a null response?)

Note that I am not a professional (I try to develop my own app) so I would really appreciate your help and, if possible, elaborated answers.

I thank you for your help !

Bernard