Thanks to Graham Hallas for alerting me to why so many people have been having some bizarre problems lately.
It seems that the link on the page https://geek.hellyer.kiwi/multi-level-navigation/ was incorrect. It was pointing to an old version of the plugin which had serious bug and contained a widget file called suckerfish_widget.php which triggered a whole series of errors when activated. I hadn’t realised that the ZIP file name stored in the WordPress plugins repository changes every time the repository is updated, hence the error.
Apologies for any problems this may have caused. In fact it’s caused me quite a few problems too, as I’ve been racking my brain trying to figure out why on earth some people were having the problems they were having. I couldn’t figure it out for the life of me.
The version which you don’t want to download is 1.0. There was also a version 1.0.1 and there is now a 1.0.2 (minor change which is being uploaded as I type) which you should download. You should also delete the widget file suckerfish_widget.php as that file doesn’t serve any purpose and will trigger errors if you try to activate the widget.