Please explain how to fix this?

<span style=”color: #444444; font-family: -apple-system, ‘system-ui’, ‘Segoe UI’, Roboto, Oxygen-Sans, Ubuntu, Cantarell, ‘Helvetica Neue’, sans-serif; font-size: medium;”>Fatal error: Uncaught Error: Call to undefined function is_product() in /usr/www/users/pabloxjrna/wp-content/plugins/oneclick-whatsapp-order/includes/buttons/wa-order-single-product.php:539 Stack trace: #0 /usr/www/users/pabloxjrna/wp-includes/class-wp-hook.php(308): wa_order_force_show_atc_button(”) #1 /usr/www/users/pabloxjrna/wp-includes/class-wp-hook.php(332): WP_Hook->apply_filters(”, Array) #2 /usr/www/users/pabloxjrna/wp-includes/plugin.php(517): WP_Hook->do_action(Array) #3 /usr/www/users/pabloxjrna/wp-includes/general-template.php(3043): do_action(‘wp_head’) #4 /usr/www/users/pabloxjrna/wp-content/themes/pitstop/header.php(8): wp_head() #5 /usr/www/users/pabloxjrna/wp-includes/template.php(783): require_once(‘/usr/www/users/…’) #6 /usr/www/users/pabloxjrna/wp-includes/template.php(718): load_template(‘/usr/www/users/…’, true, Array) #7 /usr/www/users/pabloxjrna/wp-includes/general-template.php(48): locate_template(Array, true, true, Array) #8 /usr/www/users/pabloxjrna/wp-content/themes/pitstop/page-home.php(17): get_header() #9 /usr/www/users/pabloxjrna/wp-includes/template-loader.php(106): include(‘/usr/www/users/…’) #10 /usr/www/users/pabloxjrna/wp-blog-header.php(19): require_once(‘/usr/www/users/…’) #11 /usr/www/users/pabloxjrna/index.php(17): require(‘/usr/www/users/…’) #12 {main} thrown in /usr/www/users/pabloxjrna/wp-content/plugins/oneclick-whatsapp-order/includes/buttons/wa-order-single-product.php on line 539</span>