I'm not a RedHat guy, but that looks like there is a problem with the user "backup". Did you set up a user "backup" before launching into this?
If you take a look at
http://wiki.zmanda.com, you will find a quickstart under User Documentation that outlines all the steps in getting Amanda running. That's what I used (I'm on Solaris, but the quickstart is generic), jumping off into other sections of the documentation as needed. There is also an Installation page linked from the User Documentation page that goes over more detail on the initial installation.