Do you know why the widget does not appear in the sidebar?

There will be two possible reasons:

First, you are not logged in to your wp-admin and see the sidebar without widgets. Second, you must have made some tweaks to your theme and that's why it's not showing up properly. The solution will be to deactivate and reactivate the theme and roll back to previous changes.

If the problem persists you need to reach out to the WP developer.