The plugin is compatible with WP 4.9.1.
When you activate the plugin, it creates a “Search Results” page, the slug code of this page is “search_gcse”.
This page contains the following shortcode, where the plugin displays search results: [wp_google_search]
This page has to be exist for displaying search results, and it must be a public page. Please check it in the “Pages” menu of your WP site.
If there is any problem of displaying search results, check that if this page exists, and is published.
You can deactivate the plugin then activate again for creating this page again if it was deleted.