Error PHP Notice: Undefined variable: _SESSION in /opt/bitnami/apps/wordpress/htdocs/wp-content/plugins/Lost-password/lost-passwords.php on line 57

Question

I'm very new to php and the kind of guy how follows tutorials and try to do the best of each, but I'm having some trouble with a custom reset password form on WooCommerce...

I have follow all the steps given by @Polar on WooCommerce lost password form on another page answer. But I'm not able to get the reset password form, When I activate the debug mode I got the following error:

"PHP Notice: Undefined variable: _SESSION in …/wp-content/plugins/Lost-password/lost-passwords.php on line 54"

This is what is written on line 54:

unset( $_SESSION[ "csx-show-reset-form" ] );

I'm able to put the user email and actually receive the email for reseting the password, but when I follow the link, it shows me the "insert your email" page again.

This is the code I'm using:

<?php

/*
Plugin Name:  Lost Password
Version: 1.0
Description: Personalized Shortcode for recovering password.
Text Domain: FA-Lost-Password
*/

/**
* Crea el shortcode [lost_password_form] para el formulario de los password
*/
 
function wc_custom_lost_password_form( $atts ) {
    if ( !empty( $_COOKIE[ "csx-reset-link-set" ] ) && isset( $_COOKIE[ "csx-reset-link-set" ] ) && $_COOKIE[ "csx-reset-link-set" ] === "true" ) { // WPCS: input var ok, CSRF ok.
        return wc_get_template( 'myaccount/lost-password-confirmation.php' );
    } elseif ( !empty( $_SESSION[ "csx-show-reset-form" ] ) && isset( $_SESSION[ "csx-show-reset-form" ] ) && $_SESSION[ "csx-show-reset-form" ] === "true" ) {
        $rp_id = $_SESSION[ "csx-id" ];
        $rp_key = $_SESSION[ "csx-key" ];
        if ( isset( $_COOKIE[ 'wp-resetpass-' . COOKIEHASH ] ) && 0 < strpos( $_COOKIE[ 'wp-resetpass-' . COOKIEHASH ], ':' ) ) { // @codingStandardsIgnoreLine
            list( $rp_id, $rp_key ) = array_map( 'wc_clean', explode( ':', wp_unslash( $_COOKIE[ 'wp-resetpass-' . COOKIEHASH ] ), 2 ) ); // @codingStandardsIgnoreLine
            $userdata = get_userdata( absint( $rp_id ) );
            $rp_login = $userdata ? $userdata->user_login : '';
            $user = WC_Shortcode_My_Account::check_password_reset_key( $rp_key, $rp_login );

            // Reset key / login is correct, display reset password form with hidden key / login values.
            if ( is_object( $user ) ) {
                return wc_get_template(
                    'myaccount/form-reset-password.php',
                    array(
                        'key' => $rp_key,
                        'login' => $rp_login,
                    )
                );
            }
        }
    }

    // Show lost password form by default.
    return wc_get_template(
        'myaccount/form-lost-password.php',
        array(
            'form' => 'lost_password',
        )
    );
}
add_shortcode( 'lost_password_form', 'wc_custom_lost_password_form' );

//Handling query
function csx_process_query() {

    if ( isset( $_GET[ 'reset-link-sent' ] ) && $_GET[ 'reset-link-sent' ] === "true" ) {
        setcookie( 'csx-reset-link-set', "true", time() + ( 300 * 1 ), "/" ); //Allow to submit email for reset after 5 minutes only.
        unset( $_SESSION[ "csx-show-reset-form" ] );
    }

    if ( isset( $_GET[ 'show-reset-form' ] ) && $_GET[ 'show-reset-form' ] === "true" ||
        isset( $_GET[ 'key' ] ) && isset( $_GET[ 'id' ] ) ) {
        $_SESSION[ "csx-show-reset-form" ] = "true";
        setcookie( 'csx-reset-link-set', "", time() - 3600, "/" );
    }

    //Set session and cookie if key and id are existed
    if ( isset( $_GET[ 'key' ] ) && isset( $_GET[ 'id' ] ) ) {
        $_SESSION[ "csx-key" ] = $_GET[ 'key' ];
        $_SESSION[ "csx-id" ] = $_GET[ 'id' ];

        $value = sprintf( "%s:%s", wp_unslash( $_GET[ 'id' ] ), wp_unslash( $_GET[ 'key' ] ) );
        WC_Shortcode_My_Account::set_reset_password_cookie( $value );
    }

    //Unset session and cookie after successfully changed the password.
    if ( isset( $_GET[ 'new-password-created' ] ) && $_GET[ 'new-password-created' ] === "true" ) {
        setcookie( 'wp-resetpass-' . COOKIEHASH, "", time() - 3600 );
        unset( $_SESSION[ "csx-show-reset-form" ] );
        unset( $_SESSION[ "csx-reset-link-set" ] );
        unset( $_SESSION[ "csx-id" ] );
        unset( $_SESSION[ "csx-key" ] );
    }
}
add_action( 'init', 'csx_process_query' );

//Redirect to custom lost password on request
function csx_redirections() {
    if ( isset( $_GET[ 'reset-link-sent' ] ) || isset( $_GET[ 'show-reset-form' ] ) ||
        isset( $_GET[ 'key' ] ) && isset( $_GET[ 'id' ] ) ) {
        wp_redirect( home_url() . '/password' );
        exit;
    }
}
add_action( 'template_redirect', 'csx_redirections' );

/**
*  Redirecccionar al usuario cuando la contraseña fue cambiada exitosamente
*/

function woocommerce_new_pass_redirect( $user ) {
    wc_add_notice( __( '¡Su Contraseña ha sido actualizada correctamente!', 'woocommerce' ), 'success' );
    wp_redirect( home_url() . "/mi-cuenta/?new-password-created=true" );
    exit;
}
add_action( 'woocommerce_customer_reset_password', 'woocommerce_new_pass_redirect' );

I'll apreciate any help you can give me.

Thanks