Flutter – display http request response in a Listview

i try to fix you code.

I assume the response from the API is a List ([]), because you use the data like a List as data[0].

I already commented what i add and the use of the code.

import 'dart:convert';
import 'package:flutter/material.dart';
import 'package:http/http.dart' as http;

class StfNouveauCompte extends StatefulWidget {
  const StfNouveauCompte({super.key});

  @override
  State<StfNouveauCompte> createState() => _StfNouveauCompteState();
}

class _StfNouveauCompteState extends State<StfNouveauCompte> {
  String msg = "";
  var jsonList;
  final _formKey = GlobalKey<FormState>();
  final controllernom = TextEditingController();
  final controllernconfirmationmotdepasse = TextEditingController();

  // New variable to save the data from API
  List dataFromApi = [];

  @override
  void dispose() {
    super.dispose();
    controllernom.dispose();
    controllernconfirmationmotdepasse.dispose();
  }

  // A function that return the ListView that contain the data from API
  ListView getListView() {
    return ListView.builder(
      // Use dataFromApi length
      // So if dataFromApi is empty, it will not showing anything.
      itemCount: dataFromApi.length,
      itemBuilder: (BuildContext context, int index) {
        return Card(
          key: Key('$index'),
          child: ListTile(
            // Use [dataFromApi] value
            title: Text('$dataFromApi[0]'),
          ),
        );
      },
    );
  }

  Future<void> register(String nom, String prenoms, String mail, telephone,
      naissance, pseudo, passe1, passe2) async {
    setState(() {
      msg = '';
    });
    try {
      var url = Uri.parse('https://konamicash.com/register_app');
      var response = await http.post(url, headers: {
        "Accept": "application/json",
        "Access-Control-Allow-Origin": "*"
      }, body: {
        "nom_utilisateur": nom,
        "confirmation_du_mot_de_passe": passe2,
      });
      var codes = response.statusCode;

      if (response.statusCode == 200) {

        // assign data from API
        dataFromApi = json.decode(response.body);

        // Show dataFromApi
        print(dataFromApi.length);

        // Rebuild state to inform state that [dataFromApi] is changed it's value
        setState(() {});
      } else {
        print('Error: ${response.statusCode}');
      }
    } catch (e) {
      print('An error occurred: $e');
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Namis.com'),
        backgroundColor: Colors.pink,
      ),
      body: Center(
        child: SingleChildScrollView(
          child: Form(
            key: _formKey,
            child: Column(
              crossAxisAlignment: CrossAxisAlignment.stretch,
              children: [
                Container(
                  margin: EdgeInsets.only(bottom: 15),
                  child: Text(
                    'Register',
                    style: TextStyle(
                        fontSize: 35,
                        fontWeight: FontWeight.bold,
                        color: Colors.grey.shade400),
                    textAlign: TextAlign.center,
                  ),
                ),
                Container(
                  padding: EdgeInsets.all(20),
                  margin: EdgeInsets.only(bottom: 10),
                  child: TextFormField(
                    keyboardType: TextInputType.text,
                    decoration: InputDecoration(
                      labelText: 'Nom',
                      border: OutlineInputBorder(),
                      suffixIcon: Align(
                        widthFactor: 1.0,
                        heightFactor: 1.0,
                        child: Icon(
                          Icons.person,
                        ),
                      ),
                    ),
                  ),
                ),

                Container(
                  padding: EdgeInsets.all(20),
                  margin: EdgeInsets.only(bottom: 10),
                  child: TextFormField(
                    obscureText: true,
                    keyboardType: TextInputType.text,
                    decoration: InputDecoration(
                      labelText: 'password',
                      border: OutlineInputBorder(),
                    ),
                    controller: controllernconfirmationmotdepasse,
                  ),
                ),
                SizedBox(
                  width: double.infinity,
                  height: 50,
                  child: ElevatedButton(
                    onPressed: () {
                      if (_formKey.currentState!.validate()) {
                        final nom = controllernom.text;

                        final confirmationmotdepasse =
                            controllernconfirmationmotdepasse.text;

                        FocusScope.of(context).requestFocus(FocusNode());

                        ScaffoldMessenger.of(context).showSnackBar(
                            const SnackBar(content: Text('Envoi en cours')));

                        register(
                            nom,
                            prenoms,
                            adressemail,
                            telephone,
                            datedenaissance,
                            pseudo,
                            motdepasse,
                            confirmationmotdepasse);
                      }
                    },
                    child: Text('Créer le compte'),
                  ),
                ),

                // New widget to show ListView
                // 
                // [ListView] need an expanded widget to take all the remining height,
                // since this page using [SingleChildScrollView] so we need to define the height.
                Container(
                  width: double.infinity,
                  height: 500,
                  margin: const EdgeInsets.all(16),
                  child:  getListView(),
                ),

                Row(
                  mainAxisAlignment: MainAxisAlignment.center,
                  children: [
                    Text("J'ai déjà un compte..."),
                    TextButton(
                      onPressed: () {},
                      child: Text(
                        'Je me connecte',
                        style:
                            TextStyle(color: Color.fromARGB(255, 245, 11, 11)),
                      ),
                    ),
                  ],
                ),
                //just bellow the sized box where i want to display my different message result_message
                SizedBox(
                  height: 30,
                ),
                Text(
                  msg,
                  style: TextStyle(color: Colors.red),
                ),
              ],
            ),
          ),
        ),
      ),
    );
  }
}

This is just the basic example to use the data for the widget. I suggest you to learn FutureBuilder for data fetching from API.

Hope it helps! If you have any question please just ask:) thanks.