Server Error 500

When executing the script, it gives a server error:

HTTP Error 500 (Internal Server Error): An unexpected condition was encountered while the server was attempting to fulfill the request.

What’s the problem?

If there is a need, I will post the script. It does not refer to the database.
The server is lamp-server

  require_once "conf.php";
  if (!isset($_POST['save'])){
  else {
    if (($nazv ==="") || ($opis==="") || ($cat===""){
      echo " <script type=\"text/javascript\"> alert (\"    \"); </script>";
      echo ("    ");
    if ($_FILES['myfile']['type']!="image/jpeg") {
      echo " <script type=\"text/javascript\"> alert (\"  \"); </script>";
<form method="post" enctype="multipart/form-data">
   : <br>
  <input type="text" name="nazv"><br>
   : <br>
  <input type="text" name="opis"><br>
  <input type ="text" name="cat"><br>
  <INPUT NAME=myfile TYPE=file><br><br>
  <input type="submit" name="save">
  <input type="submit" name="otmena">

Answer 1, authority 100%

If this script is copied correctly, it looks like the parenthesis is missing:

if (($nazv ==="") || ($opis==="") || ($cat==="")

This shouldn’t cause a 500 error, but still…

Answer 2

There was another mistake…. Thanks to Dmitri Kadykov for the hint to look at the logs.