Please ask your question in the below plugin support forum.
https://wordpress.org/support/plugin/add-search-to-menu/
You can update PHP version to the latest one by contacting your web host which you are using to host your website.
@lucilafreire You can configure what post types to search in the Ivory Search plugin settings as described in this page https://ivorysearch.com/docs/search-specific-post-types/
If you have more questions then feel free to ask it in the plugin support forum below.
https://wordpress.org/support/plugin/add-search-to-menu/
