missing add to cart button

The uses of wc_cart_fragments_params is j=a.ajax({url:wc_add_to_cart_params.wc_ajax_url.toString().replace("%%endpoint%%","get_variation") in javascript file add-to-cart-variation.min.js and cart-fragments.min.js

The global variable wc_cart_fragments_params will be defined as the script wc-cart-fragments is loaded. This script is globally loaded, but may not loading due to override of any plugin or theme.

Since the script wc-cart-fragments is not loading and the variable wc_cart_fragments_params generated, we are getting the error Uncaught ReferenceError: wc_cart_fragments_params is not defined.

To remove this error you can update the add-to-cart-variation.min.js changing the wc_cart_fragments_params with wc_add_to_cart_params.

or add this code to your theme

add_action( 'wp_print_scripts', 'my_localize_printed_scripts', 90 );
function my_localize_printed_scripts(){
  wp_localize_script('woocommerce', 'wc_cart_fragments_params', array('ajax_url' => WC()->ajax_url(), 'wc_ajax_url' => WC_AJAX::get_endpoint("%%endpoint%%"), 'fragment_name' => 'wc_fragments') );
}