Hello,
Take our apologies for the delay. We have a lot of requests during last days.
You are using third-party plugin to add that button – Free Downloads WooCommerce plugin. Single product builder uses just some WooCommerce hooks to re-build elements and some hooks that your third-party plugin requires could be missing. Sorry, but if you use third-party plugins for the single product page then use one of the pre-created layouts instead of the builder.
Regards