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?
So which Version of Unity and which version of VRTK are best used together?
This is quite simple fix, you can search for all references to the Unity UI namespace and replace them all with the updated namespace name.
what version of unity and VRTK are you using, you will get no help without that basic info