Add a drop down to product edit pages in product data “General” settings tab

I am trying to figure out how to modify the singe product options so the product admin can pick from the drop down list the condition of product, i.e. new/ used.

Below is a code that allows product admin to enter the condition of product manually.

// Enabling and Displaying Fields in backend
add_action( 'woocommerce_product_options_general_product_data', 'woo_add_custom_general_fields' );
function woo_add_custom_general_fields() {
    global $woocommerce, $post;

    echo '<div class="options_group">';

    woocommerce_wp_text_input( array( // Text Field type
        'id'          => '_Stan', 
        'label'       => __( 'Stan', 'woocommerce' ), 
        'placeholder' => 'i.e: nowa; uzywana...',
        'desc_tip'    => 'true',
        'description' => __( 'Podaj stan plyty.', 'woocommerce' ) 
    ) );

    echo '</div>'; // Closing </div> tag HERE
}

// Save Fields values to database when submitted (Backend)
add_action( 'woocommerce_process_product_meta', 'woo_save_custom_general_fields' );
function woo_save_custom_general_fields( $post_id ){

    // Saving "Conditions" field key/value
    $Stan_field = $_POST['_Stan'];
    if( !empty( $Stan_field ) )
        update_post_meta( $post_id, '_Stan', esc_attr( $Stan_field ) );


}
add_action('woocommerce_single_product_summary', 'woo_display_custom_general_fields_values', 45);
function woo_display_custom_general_fields_values() {
    global $product;

    echo '<p class="custom-Stan">Stan: ' . get_post_meta( $product->id, '_Stan', true ) . '</p>';
}

There is some errors and mistakes so I have revisited a little bit your code. Now you will have to replace woocommerce_wp_text_input() by woocommerce_wp_select() to get a select field instead, this way:

// Enabling and Displaying Fields in backend
add_action( 'woocommerce_product_options_general_product_data', 'woo_add_custom_general_fields' );
function woo_add_custom_general_fields() {

    echo '<div class="options_group">';

    woocommerce_wp_select( array( // Text Field type
        'id'          => '_Stan',
        'label'       => __( 'Stan', 'woocommerce' ),
        'description' => __( 'Podaj stan plyty.', 'woocommerce' ),
        'desc_tip'    => true,
        'options'     => array(
            ''        => __( 'Select product condition', 'woocommerce' ),
            'Nowa'    => __('Nowa', 'woocommerce' ),
            'Uzywana' => __('Uzywana', 'woocommerce' ),
        )
    ) );

    echo '</div>';
}

// Save Fields values to database when submitted (Backend)
add_action( 'woocommerce_process_product_meta', 'woo_save_custom_general_fields', 30, 1 );
function woo_save_custom_general_fields( $post_id ){

    // Saving "Conditions" field key/value
    $posted_field_value = $_POST['_Stan'];
    if( ! empty( $posted_field_value ) )
        update_post_meta( $post_id, '_Stan', esc_attr( $posted_field_value ) );


}

// Display In front end
add_action( 'woocommerce_product_meta_start', 'woo_display_custom_general_fields_values', 50 );
function woo_display_custom_general_fields_values() {
    global $product;

    // compatibility with WC +3
    $product_id = method_exists( $product, 'get_id' ) ? $product->get_id() : $product->id;

    echo '<span class="stan">Stan: ' . get_post_meta( $product_id, '_Stan', true ) . '</span>';
}

Code goes in function.php file of the active child theme (or active theme).

Tested and works.