I Have The Latest Unity version and when i import package called "PRO Military Customizable FPS Character v1.1_2PRO" it imports with two errors
E:/UP/FPPGame/Library/PackageCache/com.unity.package-manager-ui@1.9.11/Editor/Sources/UI/Common/PopupField.cs(8,36): error CS0246: The type or namespace name `BaseTextElement' could not be found. Are you missing an assembly reference?
E:/UP/FPPGame/Library/PackageCache/com.unity.package-manager-ui@1.9.11/Editor/Sources/UI/Common/PopupField.cs(118,33): error CS0115: `UnityEditor.PackageManager.UI.PopupField<T>.ExecuteDefaultAction(UnityEngine.Experimental.UIElements.EventBase)' is marked as an override but no suitable method found to override
And when i import the asset the package manager dissapear from window menu
what should i do ..
I had exactly the same issue on MacOS with Unity 2019.3.9f1.
Not a perfect solution, but upgrading to Unity 2019.3.10f1 instead solved it.