Products purchased from the asset store are also distributed by the asset store (outside of my control) and their delivery process has issues from time to time.
A quick search of Google with show that people have been having this issue for years and I am surprised that Unity has not implemented a better distribution system by now.
So - the quick set of things to check for are:
- You try it install your package and fails. Does your target disk have enough space to decompress the package?
FIX: Create more space on your target disk - you need at least twice the free space of the original package for this to complete properly.
- Does your Unity Project or UnityPackage exist in a directory with non-English characters in it?
FIX: Remove the non english characters.
- Do you have read\write access to the UnityPackage, its parent directory, and your Unity Project's directory?
FIX: Fix up your permissions.
If you are still having a problem then please lodge a support request with Unity at https://support.unity3d.com/hc/en-us/requests/new