Hi Donald,
The issue is, that the wordpress ajax handler URL is not correct. The plugin tries to do a request to the site main page, while it should be requesting the /wp-admin/admin-ajax.php file. I strongly recommend investigating that issue, the WordPress default ajax url should not be changed, it may conflict with other plugins as well. It is unfortunately not caused by the this plugin, because the search only requests that URL from the WordPress core.
As a temporary resolution, you can enable the custom ajax handler here: https://i.imgur.com/ZFp7fv2.png
