The final build has not been released yet. This is still 4.7 Preview 7. If you use the build, just make a copy of your project, Do not convert in place as that is always bad practice and it’s best to always have a back up of your project.
If you’ve not set a value for the sun rotation speed then the default is zero. The “Error” you see when you create the float is just there so that when you compile you can select the variable in the panel on the left and scroll to the bottom and set a value for your float variable. Otherwise it will always be 0.
If you set it to something like 5 you’ll see the sun start moving.
Give that a shot and let us know. 