Your code looks really error prone, but focusing on your issue: The exception looks quite clear, your response parsing is breaking.
(I would also say that you should wrap your code with a try/catch to prevent any code breaking and catch properly issues).

Let’s fix your issue:

var data = jsonDecode(response.body);

This converts your string (response.body) into a json, which means data is either Map<String, dynamic> or List<dynamic> (Where dynamic is a Map or another nested List), so the following makes no sense

if (data == "success") {

Now, let’s have a look at your php code:

echo json_encode("success");

I’m not a php expert, but from the documentation looks like it should be used like the following:

$response = array("result" => "Success");
echo json_encode($response);

Let’s go back to your dart code now:

    var success = false;
    try{

      final baseUrl = "http://192.168.43.150"; // Use final wherever you can
      final url = "$baseUrl/v1_flutter/lib/php/connection.php"; // Improving flexibility

      final body = <String, String>{
        "email": email.text,
        "pass": password.text,
      }; // Decouple in variables for readability

      final headers = <String, String>{"Accept":"application/json"}; // Fix typo

      final response = await http.post(Uri.parse(url), body: body, headers: headers);
      final data = jsonDecode(response.body);
      success = data["result"] == "Success";
   } catch(e) {
      print("Catched an error!");
      print(e);
      success = false;
   }

   if(success) {
   ...

php file:

 $email=$_POST['email'];

    $passworda=$_POST['passworda'];

$sql="SELECT * FROM user WHERE email='".$email."'AND passworda='".$password."' ";
$result=mysqli_query($db,$sql);
$count=mysqli_num_rows($result);
if($count>=1){
echo json_encode("success");
}
else
{
echo json_encode("error");
}

logine page Flutter:

class Login extends StatelessWidget {
  TextEditingController email = TextEditingController();
  TextEditingController password = TextEditingController();

  Future login(BuildContext cont) async {
    if (email.text == "" || password.text == "") {
      Fluttertoast.showToast(
        msg: "please complete!",
        toastLength: Toast.LENGTH_SHORT,
        gravity: ToastGravity.CENTER,
        fontSize: 16.0,
      );
    } else {
      var url = "http://192.168.43.150/v1_flutter/lib/php/connection.php";
      var response = await http.post(Uri.parse(url), body: {
        "email": email.text,
        "pass": password.text,
      }, headers: {"Accept":"applicarion/json"});
      var data = jsonDecode(response.body);

      if (data == "success") {
        Navigator.pop(cont);
        Navigator.pushNamed(cont, "/registre");
      } else {
        Fluttertoast.showToast(
          msg: "The user and password does not exist!",
          toastLength: Toast.LENGTH_SHORT,
          gravity: ToastGravity.CENTER,
          fontSize: 16.0,
        );
      }
    }}

Console:

E/flutter ( 6084): [ERROR:flutter/lib/ui/ui_dart_state.cc(198)]
Unhandled Exception: FormatException: Unexpected character (at
character 1)
E/flutter ( 6084):
E/flutter ( 6084): ^
E/flutter ( 6084):
E/flutter ( 6084): #0 _ChunkedJsonParser.fail (dart:convert-patch/convert_patch.dart:1383:5)
E/flutter ( 6084): #1 _ChunkedJsonParser.parseNumber (dart:convert-patch/convert_patch.dart:1250:9)
E/flutter ( 6084): #2 _ChunkedJsonParser.parse (dart:convert-patch/convert_patch.dart:915:22)
E/flutter ( 6084): #3 _parseJson (dart:convert-patch/convert_patch.dart:35:10)
E/flutter ( 6084): #4 JsonDecoder.convert (dart:convert/json.dart:612:36)
E/flutter ( 6084): #5 JsonCodec.decode (dart:convert/json.dart:216:41)
E/flutter ( 6084): #6 jsonDecode (dart:convert/json.dart:155:10)
E/flutter ( 6084): #7 Login.login (package:mes_v1/pages/Authentification/login.dart:25:18)
E/flutter ( 6084):
E/flutter ( 6084):