Sometimes when trying to import a freshly downloaded package you can run into the following error:
Failed to import package with error: Couldn't decompress package
This error is visible on the console often along with an empty import dialogue saying:
"Nothing to import, all assets from this package are already in your project"
A quick search on the Internet can show that people have been experiencing with all kind of asset packages in the past:
From our support experience, a list of things to check for are:
1. 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.
2. Does your Unity Project or UnityPackage exist in a directory with non-English characters in it?
FIX: Remove the non english characters in that directory.
3. Do you have read\write access to the UnityPackage, its parent directory, and your Unity Project's directory?
FIX: Assign yourself read / write permissions to these directories.
4. Are you sure the download is not corrupt? Sometimes it can happen that the downloaded file gets corrupted or is incomplete. If you have 7zip installed on your computer (note there are unofficial download options for Mac & Linux as well) , you could test this theory by opening the unitypackage file on your computer. You can find the package files in the following directory:
Windows: %APPDATA%\Unity\Asset Store-5.x (hidden folder!)
Mac: ~/Library/Unity/Asset Store-5.x/
Linux: ~/.local/share/unity3d/Asset Store-5.x
If 7zip shows you an error or an empty package when opening it, then the package is corrupt and needs to be re-downloaded. To do so, please delete this file and download it again from the asset store. We are aware that this sounds like an unlikely solution to this issue, but we are getting this issue reported about once per week and a simple redownload is often the solution.
5. In combination with the "corrupt download" issue, we had reports of security software such as Malwarebytes causing the download corruption. If a simple redownload still results in a corrupt download, it might be worth to consider deactivating additional software that might influence / scan downloads during the download until the installation is finished.
6. Try to delete your Unity Cache. Some times if you delete your cache this will also resolve the issue 9/10 times.
If you are still having a problem then please create a support request with Unity at https://unity3d.com/learn/support.