Do you have errors in the console? It usually disappear if you have some errors which occur before the package manager UI.
Yes I do
Library\PackageCache\com.unity.entities@0.0.12-preview.12\Unity.Rendering.Hybrid\MeshInstanceRendererSystem.cs(2,22): error CS0234: The type or namespace name ‘Remoting’ does not exist in the namespace ‘System.Runtime’ (are you missing an assembly reference?)
You will have to switch the Runtime Version to .net 4x Equivalent and when the editor restarts those errors should be gone. This is under Settings > Player
That was it
For something core like this, it seems a little brittle. I logged a bug #1091061
Haha, yea. Thinking about it now I have no clue what made me tried that and figured this out. It was actually the first thing I tried and it worked so I kinda just disregarded it as something everyone would know to do.