Fatal error: Call to undefined function wp_create_nonce()

More context would be helpful. Is that all the code found in your plugin or functions file directly? Or are you hooking in to something via add_action.

Anyway, what’s probably wrong is that you’re calling wp_localize_script and wp_enqueue_script outside of an action. wp_create_nonce, or, rather, the file in which it resides, has yet to be loaded.

The solution is to call wp_localize_script from inside a function hooked into wp_enqueue_scripts

<?php
add_action( 'wp_enqueue_scripts', 'wpse30583_enqueue' );
function wpse30583_enqueue()
{
    // your enqueue will probably look different.
    wp_enqueue_script( 'wpse30583_script' );

    // Localize the script
    $data = array( 
        'ajax_url' => admin_url( 'admin-ajax.php' ),
        'nonce'    => wp_create_nonce( 'wpse30583_nonce' )
    );
    wp_localize_script( 'wpse30583_script', 'wpse3058_object', $data );
}