If you download any of the new 2019.* Unity releases you may get a compiler error on some of the classes complaining about the UnityEngine.UI namespace. This seems to be a known bug within Unity and I have identified these tickets that could be the cause:
Not sure if there is a solution here.
Do you get these errors with V3 or V4?
This is a namespace change in beta/alpha versions above 2019.1.0, you will need to use a version below those for now or find all references to the namespace and rename them appropriately
What is the namespace change? Is it the UIElements package thing?