anyone can find the syntax misstake in my php?

I'm trying to write out the errors, which are created in my .php , as an alert in my .html but i cant get it to run.

<?php if(isset($_SESSION["error"])){
     echo "<script> alert(".$_SESSION["error"].");";
     echo "</script>";}
    ?>

i thnk its just a misstake in the syntax, becuase when i run it in an online php testing program it works

php is enabled in .html and its working 10 lines later :D
in this example its working:

<?php if(!isset($_SESSION["login_user"])){
                    echo "<fieldset> <form id='login' method='post' action='../php/login.php'>";
                    echo "<input type='text' name='email' placeholder='E-Mail Adresse' required>";
                    echo "<input type='password' name='passwort' placeholder='Passwort' required><br/>";
                    echo "<a href='registrierung.html'>Registrieren</a>";
                    echo "          <input name='submit' id='anmelden' type='submit' value='Login'>";
                    echo "      </form>";
                    echo "</fieldset>";
                    }
                    else{
                    //Hier Html für eingeloggt Kasten Bitte
                    }
                ?>