It has already been answered in
here.
Btw, 0x80070091 means "The directory is not empty.", presumably means the restore process attempted to wipe clean the Windows app folder, but somehow unable to do so because there's still something in there.
P.S.: I recommend you to rename the "Windows Apps" folder to something else instead of purge it with "
rd /s", just in case some preinstalled Apps cannot be re-installed, you can pull back from this
backup.