Looking at the plugin code before it made it into core: https://github.com/piersharding/moodle-tool_uploadcourse/blob/master/locallib.php#L517-L526 - it appears that the backup file needs to be on the same machine as the Moodle installation. There is no 'upload' only read a file from the local machine.
Courses and course formats
bulk course upload with backup files.
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