Thanks for letting us know, @JoelZaslofsky!
For v.2.8.0.beta10, I incorrectly pinned the plugin to a commit that relies on a Discourse core change that it’s only available on beta11. Here’s a fix:
https://github.com/discourse/discourse-reactions/pull/137
Please rebuild your site to fetch the correct plugin commit, and let us know if you see this error again.