Hi,
This is most likely caused by a view override of the product page layout. In HikaShop 6.4.0, the product page templates were restructured so that the form element is no longer wrapping the entire product page. If you have a view override of the product page layout (show_default.php or similar) from before 6.4.0, it may be missing the form element that the current version expects.
You can check this in the HikaShop backend by going to Display > Views and looking for overrides of files starting with "show_" in the product view. If you have overrides there, you can use the "diff" button to compare your override with the current default template and see what changed.
As a quick test, you can temporarily rename the folder templates/YOUR_TEMPLATE/html/com_hikashop/product/ to confirm that the default view files of the product page work correctly. If that fixes the issue, you will need to update one or several of the view overrides in that folder to match the new template structure.