Warning: mysqli_error() expects parameter 1 to be mysqli, string given in

I want to connect to a database (with xampp) called ranch and insert into this db some form data. The browser displays the error:

Warning: mysqli_error() expects parameter 1 to be mysqli, string given in C:\xampp\htdocs\project1\register.php on line 38

ERROR: Could not able to execute INSERT INTO child_parent('childname','childsurname','age','gender','name','surname','address','tk','city','telephone','mobile','email','parea','pass') VALUES('nikos','ads','34','Αγόρι','sds','sds','dsd','34','dsds','34','434','[email protected]','sds','34').

I have the .php file that is below. How can I fix it?

<?php

ini_set('display_errors', 'On');
session_start();
unset ($msg);
    echo "kajsj<br>";
    $conn= new mysqli("localhost","root","","ranch");
    
    if (mysqli_connect_errno())
    { printf("Connect failed: %s\n",mysqli_connect_error());//error message
    }
    else
    {
    printf("Connect achieved<br>"); 
    echo $_GET['childname'];
    $childname=$_GET['childname'];
    $childsurname=$_GET['childsurname'];
    $age=$_GET['age'];
    $gender=$_GET['gender'];
    $name=$_GET['name'];
    $surname=$_GET['surname'];
    $address=$_GET['address'];
    $tk=$_GET['tk'];
    $city=$_GET['city'];
    $telephone=$_GET['telephone'];
    $mobile=$_GET['mobile'];
    $email=$_GET['email'];
    $parea=$_GET['parea'];
    $pass=$_GET['pass'];
    // Insert data into mysql 
    $query1="INSERT INTO child_parent('childname','childsurname','age','gender','name','surname','address','tk','city','telephone','mobile','email','parea','pass')
                                VALUES('$childname','$childsurname','$age','$gender','$name','$surname','$address','$tk','$city','$telephone','$mobile','$email','$parea','$pass')";
    
    if(mysqli_query($conn, $query1))
    {echo "Records added successfully.";
    } 
    else{echo "ERROR: Could not able to execute $query1. " . mysqli_error($query1);
    }
    }
?>

There are three main problems with your code:

1. Wrong SQL. You have to learn basic SQL syntax prior trying to use SQL from PHP.
2. Wrong mysqli syntax. You have to use prepared statements instead of adding variables right in the query.
3. Wrong error reporting. You have to set mysqli in exception mode instead of checking every query result manually.

To solve them one by one, read these three helpful answers:

1. When to use single quotes, double quotes, and backticks in MySQL
2. How can I prevent SQL injection in PHP?
3. How to get mysqli error in different environments?