I found there is a built-in "tar" command in Windows cmd, and seemingly it will mask the "tar" in Rtools. It may be this one that failed to untar the downloaded content.
When type "tar" in the cmd on my Win laptop, the message shown is different from what the GNU "tar" shows.
tar.exe: Must specify one of -c, -r, -t, -u, -x
I am wondering if anyone with a Windows 10 device found similar errors before?