I've seen a lot of people in chat wondering why they were getting so many errors importing the Prototype materials when using the most recent versions of Unity. While of course using Unity 2018.4 is one solution, I've been able to get the prototypes working fine in 2019.4 with just one minor fix, even after going through the whole Update Packages thing. It turns out that most, if not all, the problems with using the projects in the most recent version of Unity are because of the old Package Manager; just remove that and everything seems to work fine. To remove the old Package Manager, just follow these steps after opening your project in Unity:
From the top menu, go to Window -> Package Manager
A window should come up listing packages. There are several dropdowns at the top; the second dropdown should say "My Assets" (at least, that's what it says for me when I first open it); click on that dropdown and choose "In Project".
The list of packages should change; it will list about a dozen packages that are included in the prototype project. One of those packages should be "Package Manager UI". Click on that package, and then click on the "Remove" button in the lower right.
Once the old "Package Manager UI" package is removed, all the errors should disappear — at least they do for me. (That's not including any errors you may have introduced if you already started writing your own code, of course.)