Don’t expect plugins will work before the final release. This is one reason why it’s called “Preview”.
You could compile the plugin on your own, but I don’t know if there are breaking changes in code…
And you’ll get the best answer in the True Sky thread 
https://forums.unrealengine.com/community/community-content-tools-and-tutorials/15332-released-truesky-alpha-for-ue4-complete-sky-and-weather-system