How to install jquery dotdotdot plugin for WordPress?

I checked out that plugin. It’s not a WordPress plugin, so that error is only normal. Rather, it’s a jquery plugin. You would include it as part of a (child) theme or plugin. Unzip it and put the files in the src directory it in a subdirectory of your theme/plugin.

Then use wp_enqueue_script to include it in your project in stead of putting it directly in the head of the page, as the site you link to instructs. That link also shows how to use the script in your pages.

UPDATE after comments

In the unzipped folder find the file jquery.dotdotdot.js. Put it in a folder called js in your child theme.

In your child theme’s functions.php add:

add_action ('wp_enqueue_scripts','wpse240448_dotdotdot');
function wpse240448_dotdotdot () {
  wp_register_script('dotdotdot', get_template_directory_uri() . '/js/jquery.dotdotdot.js', array('jquery'));
  wp_enqueue_script('dotdotdot');
  $script="$(document).ready(function() {
        $("#wrapper").dotdotdot({
        //  configuration goes here according to your link
    });
   });
  "; 
  wp_add_inline_script ('dotdotdot',$script);
  }