I also have this error in WooCommerce / Status / Logs.
WordPress 5.5.3, Woocommerce Version 4.7.1
2020-12-19T13:36:05+00:00 CRITICAL Uncaught Error: Using $this when not in object context in /home/site/public_html/wp-content/plugins/woocommerce-ajax-add-to-cart-for-variable-products/woocommerce-ajax-add-to-cart-variable-products.php:153
Stack trace:
#0 /home/site/public_html/wp-includes/class-wp-hook.php(287): woocommerce_add_to_cart_variable_rc_callback('')
#1 /home/site/public_html/wp-includes/class-wp-hook.php(311): WP_Hook->apply_filters('', Array)
#2 /home/site/public_html/wp-includes/plugin.php(478): WP_Hook->do_action(Array)
#3 /home/site/public_html/wp-admin/admin-ajax.php(191): do_action('wp_ajax_nopriv_...')
#4 {main}
thrown in /home/site/public_html/wp-content/plugins/woocommerce-ajax-add-to-cart-for-variable-products/woocommerce-ajax-add-to-cart-variable-products.php on line 153