The plugin does not allow you to override the form-checkout.php template from the theme files.
This is because it’s a customized template and if your theme already uses a different template, the plugin will not work.
Many users, however, would like to copy the template from the plugin folder and paste it into the theme, so it can be customized.
To override the template, then, add the following code into the functions.php file:
add_filter (‘yith_wcms_load_checkout_template_from_plugin’, ‘__return_false’);
Please, note: after this change, the plugin template will be ignored and the one of the theme will be loaded. Such customization is valid ONLY for the form-checkout.php template, other templates can be overriden without adding any custom code in the functions.php of the theme.