Error from mysql in android studio

package com.complainprotectioncell;
import android.app.ProgressDialog;
import android.content.Intent;
import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.text.TextUtils;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Spinner;
import android.widget.TextView;
import android.widget.Toast;
import org.json.JSONArray;
import java.util.ArrayList;
import java.util.HashMap;
public class Register2 extends AppCompatActivity {
//    public static final String URL = "https://interactivebookfyp.000webhostapp.com/user/signUp.php";
//    AsyncHttpClient client = new AsyncHttpClient();
Button regBtn,login;
    EditText First_Name;
    String F_Name_Holder;
    String finalResult ;
    String HttpURL = "https://www.zaras360.com/panel/Complain#.php";
    Boolean CheckEditText;
    ProgressDialog progressDialog;
    HashMap<String,String> hashMap = new HashMap<>();
    HttpParse httpParse = new HttpParse();
    SessionManager sessionManager;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.register);
        this.setTitle("Registration");
        //Initializing Variables/Objects
        //Assign Id'S
        First_Name = (EditText)findViewById(R.id.f_name);
        regBtn = (Button)findViewById(R.id.BtnGo);
        login = (Button)findViewById(R.id.BtnGo1);
        //Adding Click Listener on button.
        regBtn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                CheckEditTextIsEmptyOrNot();
                if(CheckEditText){
                    // If EditText is not empty and CheckEditText = True then this block will execute.
                    UserRegisterFunction2(F_Name_Holder);
                }
                else {
                    // If EditText is empty then this block will execute .
                    Toast.makeText(Register2.this, "Please fill all form fields.", Toast.LENGTH_LONG).show();
                }
            }
        });
        login.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent myIntent = new Intent(Register2.this,
                        Login.class);
                startActivity(myIntent);
            }
        });
    }
    public void CheckEditTextIsEmptyOrNot(){
        F_Name_Holder = First_Name.getText().toString();
        if(TextUtils.isEmpty(F_Name_Holder))
        {
            CheckEditText = false;
        }
        else {

            CheckEditText = true ;
        }
    }
    public void UserRegisterFunction2(final String F_Name){
        class UserRegisterFunctionClass2 extends AsyncTask<String,Void,String> {
            @Override
            protected void onPreExecute() {
                super.onPreExecute();
                progressDialog = ProgressDialog.show(Register2.this,"Loading Data",null,true,true);
            }
            @Override
            protected void onPostExecute(String httpResponseMsg) {
                super.onPostExecute(httpResponseMsg);
                progressDialog.dismiss();
                Toast.makeText(Register2.this,httpResponseMsg.toString(), Toast.LENGTH_LONG).show();
                //Toast.makeText(Register.this,email, Toast.LENGTH_LONG).show();
                Intent in = new Intent(Register2.this,PhoneAuthActivity.class);
                startActivity(in);
            }
            @Override
            protected String doInBackground(String... params) {
                hashMap.put("f_name",params[0]);
                finalResult = httpParse.postRequest(hashMap, HttpURL);
                return finalResult;
            }
        }
        UserRegisterFunctionClass2 userRegisterFunctionClass = new UserRegisterFunctionClass2();
        userRegisterFunctionClass.execute(F_Name);
    }
}

<?php
if($_SERVER['REQUEST_METHOD']=='POST'){

include 'DatabaseConfig.php';

 $con = mysqli_connect($HostName,$HostUser,$HostPass,$DatabaseName);

 $F_name = $_POST['f_name'];
 $L_name = $_POST['L_name'];
 $email = $_POST['email'];
 $password = $_POST['password'];


//$Sql_Query = "INSERT INTO users (username,password,CNIC,email,active,first_name,last_name,phone) values ('','$L_name','$email','$password','null','null','null','null')";
$Sql_Query = "ALTER TABLE `complain` AUTO_INCREMENT = $F_name + 1";

 if(mysqli_query($con,$Sql_Query))
{
 echo 'Registration Successfully';
}
else
{
 echo 'Something went wrong';
 }
 }
}
 mysqli_close($con);
?>