Troubleshoot Pi server to backup Windows 10 machine

[Thomas Ellison]

Forgive me in advance for any of my ignorance here. I'm a noob to this. I
have a headless Raspberry Pi on my LAN that I just set up with Ubuntu
server 22.04 LTS, xrdp,and xfce that has an external USB hard drive
connected and mounted to it. I currently have it successfully set up where
I can SSH and RDP with it and have its external hard drive mapped as a
network drive on my Windows 10 machine. I also have successfully set up my
Pi where it will SSH to my Windows 10 machine. What I'm trying to do to no
avail is have the Pi where it will eventually via cron job remotely call up
my Windows machine and backup any new contents of a folder on its desktop
to a folder on the external drive of the Pi.

I've installed rdiff backup on both the Pi and Windows machine. Both have
version 2.0.5. So far after connecting via RDP to my Pi and opening a
terminal I've used the following command with no success.

rdiff-backup -v9 --print-statistics
[user]@[windows-machine-hostname].local::c:/Users/[user]/Desktop/test
/mnt/pi_hdd/test

After entering the password for connection this is the output

'rdiff-backup' is not recognized as an internal or external command,
> operable program or batch file.
> 2022-12-13 21:29:46.509029 -0500  <CLIENT-2910>  Fatal Error: Truncated
> header string (problem probably originated remotely)
>
> Couldn't start up the remote connection by executing
>
>     ssh -C [user]@[windows-machine-hostname].local rdiff-backup --server
>
> Remember that, under the default settings, rdiff-backup must be
> installed in the PATH on the remote system.  See the man page for more
> information on this.  This message may also be displayed if the remote
> version of rdiff-backup is quite different from the local version (2.0.5).
> 2022-12-13 21:29:41.903408 -0500  <CLIENT-2910>  Using rdiff-backup
> version 2.0.5
> 2022-12-13 21:29:41.903618 -0500  <CLIENT-2910>   with cpython
> /usr/bin/python3 version 3.10.6
> 2022-12-13 21:29:41.911911 -0500  <CLIENT-2910>   on
> Linux-5.15.0-1021-raspi-aarch64-with-glibc2.35, fs encoding utf-8
> 2022-12-13 21:29:41.914347 -0500  <CLIENT-2910>  Executing ssh -C
> [user]@[windows-machine-hostname].local rdiff-backup --server
> 2022-12-13 21:29:41.917329 -0500  <CLIENT-2910>  Client sending (0):
> ConnectionRequest: Globals.get with 1 arguments
> 2022-12-13 21:29:41.917652 -0500  <CLIENT-2910>  Client sending (0):
> version
>

I thought maybe I need to use the remote-schema to for the
C:\Windows\System32\OpenSSH\ssh.exe but, I'm not sure on the syntax to
include that. Is what I'm wanting to accomplish even possible? Thanks in
advance