Copy and paste this:
/system scheduler add name="backup_userman_database" on-event="/tool user-manager database save name=userman" \
start-date=jan/01/1970 start-time=15:00:00 interval=1d comment="" \
disabled=no
This should save your user-manager database everyday at 15:00 to a file named "userman.umb". You could restore this database using the "/tool user-manager database load name=userman" command.
Explanation: The originally posted command (starting directly with "add ..." will only work if you are at the "/system scheduler" hierarchy of the console, i.e. your console prompt does already show " system scheduler>"...
Best regards,
Christian Meis