Downloaded the CTFRTSSamples Unity package file from https://opsive.com/downloads/?pid=803. When I run it, Unity throws the following error: 'Failed to import package with error: Couldn't decompress package'.
I just tested the download and was able to import. Are you using Firefox? Sometimes Firefox downloads it as a tar file instead, but if you use a different browser it'll work.
I did get it to work. I have Chrome, on Windows 10, by the way, and double clicking the downloaded Unity package file is what leads to the error described above. But, if instead of double clicking on the file, I instead do it "manually" inside Unity by going to Assets -> Import Package -> Custom Package, then it works and I'm able to import it.
I'm having the same problem, but does not work however I import it, and downloading from either Chrome or Edge. The tutorial package imports fine so something to do with this file or its size?