In order to be able to install plugins from the plugin gallery directly, you need to install the “tar” command on your server. It’s a utility that unpacks archives, sort of like unzip. I’m not really sure what that will look like on a Windows server though.
If you want, you can also install plugins by downloading the archive directly to your PC, unpacking it there, and then FTPing it up to your server, in the correct plugins/ subdirectory. Then you can install it via the plugin grid.
Cheers,
Jason