by Ken Task.
The unzipped files of course backup are in a directory. Correct?
The .mbz file you create has to be done from inside that directory.
Moodle expects to find a moodle_backup.xml at the root of the extracted .mbz file. That's the road map for restoring the course contents. If it can't find that moodle_backup.xml file then you get exactly the error you are seeing.
'SoS'. Ken