from the code you have, i will make changes. copy the code below to replace your final code
if(isset($_POST['create_record'])){
// αντιστοίχηση πεδίων με τις μεταβλητές για τον πίνακα 'truck_records'
$user_id = mysqli_real_escape_string($connection, $_POST['user_id']);
$user_ip = mysqli_real_escape_string($connection, $_POST['user_ip']);
$company_name = mysqli_real_escape_string($connection, $_POST['company_name']);
$nick_enable = mysqli_real_escape_string($connection, $_POST['nick_enable']);
$company_nick_name = mysqli_real_escape_string($connection, $_POST['company_nick_name']);
$driver_kind = mysqli_real_escape_string($connection, $_POST['driver_kind']);
$driver_name = mysqli_real_escape_string($connection, $_POST['driver_name']);
$invoice = mysqli_real_escape_string($connection, $_POST['invoice']);
$shipping_note = mysqli_real_escape_string($connection, $_POST['shipping_note']);
$invoice_number = mysqli_real_escape_string($connection, $_POST['invoice_number']);
$invoice_amount = mysqli_real_escape_string($connection, $_POST['invoice_amount']);
$comments = mysqli_real_escape_string($connection, $_POST['comments']);
// αντιστοίχηση πεδίων με τις μεταβλητές για τον πίνακα 'truck_weights'
$weight_tr_full = mysqli_real_escape_string($connection, $_POST['weight_tr_full']);
$weight_tr_empty = mysqli_real_escape_string($connection, $_POST['weight_tr_empty']);
$weight_product = mysqli_real_escape_string($connection, $_POST['weight_product']);
$weight_tr_full_date = mysqli_real_escape_string($connection, $_POST['weight_tr_full_date']);
$weight_tr_empty_date = mysqli_real_escape_string($connection, $_POST['weight_tr_empty_date']);
// αντιστοίχηση πεδίων με τις μεταβλητές για τον πίνακα 'material_weight'
$material_1 = mysqli_real_escape_string($connection, $_POST['material_1']);
$material_wei_1 = mysqli_real_escape_string($connection, $_POST['material_wei_1']);
$material_2 = mysqli_real_escape_string($connection, $_POST['material_2']);
$material_wei_2 = mysqli_real_escape_string($connection, $_POST['material_wei_2']);
$material_3 = mysqli_real_escape_string($connection, $_POST['material_3']);
$material_wei_3 = mysqli_real_escape_string($connection, $_POST['material_wei_3']);
$material_4 = mysqli_real_escape_string($connection, $_POST['material_4']);
$material_wei_4 = mysqli_real_escape_string($connection, $_POST['material_wei_4']);
$material_5 = mysqli_real_escape_string($connection, $_POST['material_5']);
$material_wei_5 = mysqli_real_escape_string($connection, $_POST['material_wei_5']);
$material_6 = mysqli_real_escape_string($connection, $_POST['material_6']);
$material_wei_6 = mysqli_real_escape_string($connection, $_POST['material_wei_6']);
$material_7 = mysqli_real_escape_string($connection, $_POST['material_7']);
$material_wei_7 = mysqli_real_escape_string($connection, $_POST['material_wei_7']);
$material_8 = mysqli_real_escape_string($connection, $_POST['material_8']);
$material_wei_8 = mysqli_real_escape_string($connection, $_POST['material_wei_8']);
$material_9 = mysqli_real_escape_string($connection, $_POST['material_9']);
$material_wei_9 = mysqli_real_escape_string($connection, $_POST['material_wei_9']);
$material_10 = mysqli_real_escape_string($connection, $_POST['material_10']);
$material_wei_10 = mysqli_real_escape_string($connection, $_POST['material_wei_10']);
$sql_record = "INSERT INTO `truck_records` (
user_id, user_ip, company_name, nick_enable, company_nick_name, driver_kind, driver_name,
invoice, shipping_note, invoice_number, invoice_amount, comments
)VALUES(
'$user_id', '$user_ip', '$company_name', '$nick_enable', '$company_nick_name', '$driver_kind', '$driver_name',
'$invoice', '$shipping_note', '$invoice_number', '$invoice_amount', '$comments')";
$sql = new mysqli($host, $username, $password, $database_name); //use this connection here. asign your host, username, password, database_name to the the variables.
$result_record = $connection->query($sql_record);
$last_insert_id = $connection->insert_id;
$sql_weight = "INSERT INTO `truck_weights` (
truck_weight_record_id,
user_id,
weight_tr_full,
weight_tr_empty,
weight_product,
weight_tr_full_date,
weight_tr_empty_date
)VALUES(
'$last_insert_id',
'$user_id',
'$weight_tr_full',
'$weight_tr_empty',
'$weight_product',
'$weight_tr_full_date',
'$weight_tr_empty_date')";
$connection->query($sql_weight);
$sql_material = "INSERT INTO `material_weight` (
mat_record_id,
material_weight,
material_name
)VALUES(
'$last_insert_id',
'$material_wei_1',
'$material_1')";
$connection->query($sql_material);
if($result_record){
$smsg = 'WORK. ';
}else{
$fmsg = 'NOT WORK. ';
}
echo $fmsg . "<br>";
echo $sql_record . "<br>";
echo $sql_weight . "<br>";
echo $sql_material . "<br>";
}