Update: this has been fixed in Unity 2019.1.4f1
We are currently looking into a known issue where errors like the following are displayed after importing a Vuforia sample from the Asset Store into Unity 2019.1: Library/PackageCacheemail@example.com/Editor/Sources/UI/PackageDetails.cs(5,32): error CS0234: The type or namespace name 'UIElements' does not exist in the namespace 'UnityEngine.Experimental' (are you missing an assembly reference?) The reason for this is that Unity 2019.1 does not correctly upgrade internal package dependencies when importing a complete project pack from the Asset Store.
Until we have a fix, this can be worked around with one of the following ways:
After importing the sample into Unity 2019.1, edit your Packages/manifest.json file and set the correct version of the included package manager ui package: "com.unity.package-manager-ui": "2.1.2" First import the Vuforia sample into Unity 2018.3, and then open it with 2019.1. In this case Unity will correctly update the package reference. Thanks, Vuforia Engine Support