This error has a few different causes, all relating to the Unity editor itself.
- Potential reason 1: Unity caches your packages in a hidden directory in your profile on your main operating system disk. Does your disk have enough space free to download the package?
FIX: Create more space and try reimporting your package. You need at least twice as much space as the unpackaged file takes up.
- Potential reason 2: Your Unity Project or UnityPackage exist in a directory with non-English characters in it.
FIX: Remove the non english characters. from the project and package directories.
- Potential reason 3: You do not have read\write access to the UnityPackage, its parent directory, and your Unity Project's directory.
FIX: Fix up your permissions.
If none of the suggested fixes above solve the problem, please contact Unity support directly, or open a Unity ticket here: https://support.unity3d.com/hc/en-us/requests/new