You will be able to downvote, but if the community in question is hosted on an instance that has disabled downvotes, your downvote won’t federate to anyone else. The only people who see it will be people on the same instance as you.

For users of an instance that has disabled downvotes, their timelines silently drop all downvotes, so those users see the fediverse without the impact of any downvotes, irrelevant of where they came from or what other people see.