This is the error displayed when I right click on Demo Database (10-0) and chose task and back up. Please I need you.
I know, but that's why you get the error.
When running your backup task, then you should press Remove, to remove the current backup file in SQL. It was created in a format you can only restore and should not overwrite.
You need to create a new *.bak to store your own backup.
I found a link that describes your issue: