WordPress.com is a commercial enterprise that provides hosting for WordPress sites. The Business or eCommerce plans within WordPress.com are the only ones that allow you to install plugins on your site.
If you are a WordPress.com user with a Business or eCommerce plan, you can use Nelio A/B Testing because you are able to install the plugin on your site.
Otherwise, if you feel like Nelio A/B Testing is a must-have and would benefit users on WordPress.com, we invite you to let them know your opinion in the ideas forum.