Ciao
probably the problem lies in the default root of the ssh service active on the backup target windows server
You should make the following change on the windows server
added below into C:\ProgramData\ssh\sshd_config work for me, the only thing is need restart service "OpenSSH SSH Server"
Match User <user>
ChrootDirectory "<pathroot>"
Source of information:
https://github.com/PowerShell/Win32-OpenSSH/issues/730#issuecomment-977616358
Where for user put the user you do SFTP with and in pathroot put d:\NWBACKUP
The error you see is that the SFTP tries to create a D $ directory where it cannot (the D $ is not the system share as you think)
I have tried and it works