How to fix Warning from: Notice: Undefined index when saving form

<?php
session_start();

/* Inicia validacion del lado del servidor */
if (empty($_POST['sheets'])) {
$errors[] = "Páginas vacío";
} else if (empty($_POST['title'])) {
$errors[] = "Asunto vacío";
} else if (
    !empty($_POST['sheets']) &&
    !empty($_POST['title'])
) {
include "../config/config.php"; //Contiene funcion que conecta a la base de datos        

$title = $_POST["title"];
$description = $_POST["description"];
$sender_id = $_POST["sender_id"];
$kind_id = $_POST["kind_id"];
$user_id = $_SESSION["user_id"];
$project_id = $_POST["project_id"];
$category_id = $_POST["category_id"];
$priority_id = $_POST["priority_id"];
$status_id = $_POST["status_id"];
$attach_id = $_POST["attach_id"];
$paginas = $_POST["sheets"];
$created_at = "NOW()";
$filecase = $_POST["filecase_id"];
$way = $_POST["way_id"];
$received = $_POST["received"];
$user_received = $_POST['user_received'];
$file = empty($_POST["file"]) ? "" : $_POST["file"];
$craft = $_POST['craft'];
$datecraft = $_POST['craftdatetimepicker'];

if ((isset($_POST['mostrar']) != 1) && (isset($_POST['mostrar1']) != 1 )) {

    $query = mysqli_query($con, "SELECT COUNT(*) as cont FROM document WHERE year(created_at)=year(now())");

    if ($row = mysqli_fetch_array($query)) {
        $cont = $row['cont'] + 1; // incrementa   
        $num = sprintf("%'.05d", $cont); // formato con 5 digitos
        $codigo = date("Y") . "-" . $num; // se concatena con el año actual
    }

    $code = mysqli_query($con, "SELECT * FROM category WHERE id = $category_id");

    if ($f = mysqli_fetch_array($code)) {
        $cont1 = $f['code'];
        $codigo1 = $num . "-" . date("Y") . "-" . $cont1; // se concatena con el año actual        
    }

    if (isset($_POST["user_id"])) {
        $sel = '';
        foreach ($_POST["user_id"] as $row) {
            $sel .= $row . ', ';
        }
        $sel = substr($sel, 0, -2);
    }
    
    if (isset($_POST["project_id"])) {
        $project = '';
        foreach ($_POST["project_id"] as $row) {
            $project .= $row . ', ';
        }
        $project = substr($project, 0, -2);
    }

    $sql = "insert into document (consecutive,file,filecase_id,title,description,sender_id,kind_id,user_id,project_id,category_id,assigned_to,priority_id,status_id,attach_id,sheets,created_at, way_id, received, user_received) value (\"$codigo\",\"$codigo1\",\"$filecase\",\"$title\",\"$description\",\"$sender_id\",$kind_id,$user_id,\"$project\",\"$category_id\",\"$sel\",$priority_id,$status_id,$attach_id,$paginas,$created_at, $way, \"$received\", \"$user_received\")";
} elseif ((isset($_POST['mostrar']) == 1) && (isset($_POST['mostrar1']) != 1 )) {

    $check_new = 0;

    $query = mysqli_query($con, "SELECT COUNT(*) as cont FROM document WHERE year(created_at)=year(now())");

    if ($row = mysqli_fetch_array($query)) {
        $cont = $row['cont'] + 1; // incrementa   
        $num = sprintf("%'.05d", $cont); // formato con 5 digitos
        $codigo = date("Y") . "-" . $num; // se concatena con el año actual
    }

    $code1 = mysqli_query($con, "SELECT * FROM document WHERE id = $file");

    if ($f = mysqli_fetch_array($code1)) {
        $cont2 = $f['file'];
    }

    if (isset($_POST["user_id"])) {
        $sel = '';
        foreach ($_POST["user_id"] as $row) {
            $sel .= $row . ', ';
        }
        $sel = substr($sel, 0, -2);
    }
    
    if (isset($_POST["project_id"])) {
        $project = '';
        foreach ($_POST["project_id"] as $row) {
            $project .= $row . ', ';
        }
        $project = substr($project, 0, -2);
    }

    $sql = "insert into document (consecutive,file,filecase_id,title,description,sender_id,kind_id,user_id,project_id,category_id,assigned_to,priority_id,status_id,attach_id,sheets,created_at, way_id, received, user_received, check_new) value (\"$codigo\",\"$cont2\",\"$filecase\",\"$title\",\"$description\",\"$sender_id\",$kind_id,$user_id,\"$project\",\"$category_id\",\"$sel\",$priority_id,$status_id,$attach_id,$paginas,$created_at, $way, \"$received\", \"$user_received\", \"$check_new\")";
} elseif ((isset($_POST['mostrar']) != 1) && (isset($_POST['mostrar1']) == 1 )) {

    $query = mysqli_query($con, "SELECT COUNT(*) as cont FROM document WHERE year(created_at)=year(now())");

    if ($row = mysqli_fetch_array($query)) {
        $cont = $row['cont'] + 1; // incrementa   
        $num = sprintf("%'.05d", $cont); // formato con 5 digitos
        $codigo = date("Y") . "-" . $num; // se concatena con el año actual
    }

    $code = mysqli_query($con, "SELECT * FROM category WHERE id = $category_id");

    if ($f = mysqli_fetch_array($code)) {
        $cont1 = $f['code'];
        $codigo1 = $num . "-" . date("Y") . "-" . $cont1; // se concatena con el año actual        
    }

    if (isset($_POST["user_id"])) {
        $sel = '';
        foreach ($_POST["user_id"] as $row) {
            $sel .= $row . ', ';
        }
        $sel = substr($sel, 0, -2);
    }
    
    if (isset($_POST["project_id"])) {
        $project = '';
        foreach ($_POST["project_id"] as $row) {
            $project .= $row . ', ';
        }
        $project = substr($project, 0, -2);
    }

    $sql = "insert into document (consecutive,file,filecase_id,title,description,sender_id,kind_id,user_id,project_id,category_id,assigned_to,priority_id,status_id,attach_id,sheets,created_at, way_id, received, user_received, craft, datecraft) value (\"$codigo\",\"$codigo1\",\"$filecase\",\"$title\",\"$description\",\"$sender_id\",$kind_id,$user_id,\"$project\",\"$category_id\",\"$sel\",$priority_id,$status_id,$attach_id,$paginas,$created_at, $way, \"$received\", \"$user_received\", \"$craft\", \"$datecraft\")";
} else {

    $check_new = 0;

    $query = mysqli_query($con, "SELECT COUNT(*) as cont FROM document WHERE year(created_at)=year(now())");

    if ($row = mysqli_fetch_array($query)) {
        $cont = $row['cont'] + 1; // incrementa   
        $num = sprintf("%'.05d", $cont); // formato con 5 digitos
        $codigo = date("Y") . "-" . $num; // se concatena con el año actual
    }

    $code1 = mysqli_query($con, "SELECT * FROM document WHERE id = $file");

    if ($f = mysqli_fetch_array($code1)) {
        $cont2 = $f['file'];
    }

    if (isset($_POST["user_id"])) {
        $sel = '';
        foreach ($_POST["user_id"] as $row) {
            $sel .= $row . ', ';
        }
        $sel = substr($sel, 0, -2);
    }
    
    if (isset($_POST["project_id"])) {
        $project = '';
        foreach ($_POST["project_id"] as $row) {
            $project .= $row . ', ';
        }
        $project = substr($project, 0, -2);
    }

    $sql = "insert into document (consecutive,file,filecase_id,title,description,sender_id,kind_id,user_id,project_id,category_id,assigned_to,priority_id,status_id,attach_id,sheets,created_at, way_id, received, user_received, check_new, craft, datecraft) value (\"$codigo\",\"$cont2\",\"$filecase\",\"$title\",\"$description\",\"$sender_id\",$kind_id,$user_id,\"$project\",\"$category_id\",\"$sel\",$priority_id,$status_id,$attach_id,$paginas,$created_at, $way, \"$received\", \"$user_received\", \"$check_new\", \"$craft\", \"$datecraft\")";
}

$query_new_insert = mysqli_query($con, $sql);

if ($query_new_insert) {
    $messages[] = "Tu documento ha sido ingresado satisfactoriamente.";
} else {
    $errors [] = "Lo siento algo ha salido mal intenta nuevamente." . mysqli_error($con);
}
} else {
$errors [] = "Error desconocido.";
}

if (isset($errors)) {
?>
<div class="alert alert-danger" role="alert">
    <button type="button" class="close" data-dismiss="alert">&times;</button>
    <strong>Error!</strong> 
<?php
foreach ($errors as $error) {
    echo $error;
}
?>
</div>
    <?php
}

if (isset($messages)) {
    ?>
<div class="alert alert-success" role="alert">
    <button type="button" class="close" data-dismiss="alert">&times;</button>
    <strong>¡Bien hecho!</strong>
<?php
foreach ($messages as $message) {
    echo $message;
}
?>
</div>
    <?php
}
?>