WooCommerce – Call to undefined function is_woocommerce()

new here and to wordpress plugins development, go easy on me 😀

Anyway, I am trying to create a new plugin and I am getting a 500 error.
I changed WP_DEBUG in config.php to true in order to see what is causing the 500 error and got this message:

Fatal error: Uncaught Error: Call to undefined function is_woocommerce() in...

This is my code currently:

<?php
/**
 * Plugin Name: 
 * Plugin URI: 
 * Description: 
 * Author: 
 * Author URI: 
 * Version: 1.0
 * Text Domain: 
 *
 * Copyright: (c) 2018
 *
 * License: 
 * License URI: 
 *
 * @author    
 * @copyright Copyright (c) 2018
 * @license   
 *
 */
//
defined( 'ABSPATH' ) or exit;
if (function_exists(is_woocommerce())) {
    echo "test: ".is_woocommerce();
} else {
    echo "test: Function does not exists!";
}

If you need any more information, tell me and I’ll edit the question.
Help will be appreciated, thanks!

If you want to check one Plugin’s Function / Class etc. from another Plugin, then it’s best to use a hook like plugins_loaded.

Based on this, your Plugin CODE will look like:

<?php
/*
Plugin Name: YOUR PLUGIN NAME
*/

defined( 'ABSPATH' ) or exit;

add_action( 'plugins_loaded', 'plugin_prefix_woocommerce_check' );
    function plugin_prefix_woocommerce_check() {
    if( function_exists( 'is_woocommerce' ) ) { 
        add_action( "wp_footer", "wpse_woocommerce_exists" );
    }   
    else {
        add_action( "wp_footer", "wpse_woocommerce_doesnt_exist" );
    }   
}   

function wpse_woocommerce_exists() {
    echo "<h1>WooCommerce Exists!</h1>";
}   

function wpse_woocommerce_doesnt_exist() {
    echo "<h1>WooCommerce Doesn't Exists!</h1>";
}

Directly checking other plugin functions will often lead to error, since WordPress may not be done loading other plugins by the time your CODE executes. However, when WordPress is done, it’ll fire the plugins_loaded hook.

Check Plugin Development Guide for more information on how to develop a WordPress Plugin.