The reason is that the plugin is incompatible with PHP8. I have contacted the developer and their advice is:
At the moment, we are not quite ready with PHP 8 compatibility, but it is coming soon on the horizon. It looks like the specific issue you’re having is related to PHP 8, so unfortunately you will need to have your server set to run PHP 7.X
Update:
If you want to downgrade your PHP version from 8 to 7.4 for example, you can use this article.