errors – Drop-down menu problem on the homepage only?

Gday gentlemen,

I am new to the web and have been managing a site for a friend for a while. The latest version based on WordPress has been in place for over 2 years without any major problems.

However, I am not sure that this is due to any of the various updates over time. Recently, I noticed that the site's menu was not working properly. The items in the drop-down menu do not appear. This problem only affects the home page and none of the other pages.

Initially, the menu was (incorrectly) aligned at the bottom of the page, which was very wrong. I've used these instructions to put it back up:
https://blog.hubspot.com/website/sticky-menu-wordpress

This managed to put the menu back in the foreground, but the current problem has arisen.

Can someone take a look and advise you please? Your help is very much appreciated.
http://www.singaporevirtualairlines.org
WordPress 5.2.4 (self-hosted)
Impreza latest version Oct 30 19