Yes, plugins on official hosting are limited by tier as @Moin said. You can check the page she linked to see which plugins are supported under which tier, but the Free tier is intentionally limited. If you really need that plugin you will need to self-host or buy another hosting tier.
For your “one post per user” thing, I’m not entirely sure that restricted replies is the right thing for you regardless, as that is primarily designed for restricting replies to people in certain groups.