Please see:
https://docs.moodle.org/405/en/Course_restore#Restore_via_CLI_for_administrators
doesn't show any passphrase for cli course restore.
If the backup file was created by CLI and the original user used encryption to create the .mbz file, you would need to get the passphrase from the person who created the .mbz
To see if I am correct ... copy the .mbz file to a test directory, and try to uncompressed it ... an .mbz is a tar.gz (gunzipped) file.
Not sure what the command would be on Windows platform, but if it were on linux:
tar -xvf nameofbackupfile.mbz
If you get the same prompt, it's been encrypted and you'll need to look for a hacker tool to un-encrypt the file.
Good luck!
'SoS', Ken