Same here :)
- your script says "switch user to baccuppc and run /bin/tar"
- your sudoers file says that "backuppc" would be allowed to switch user to root and run /bin/tar"
but there is nothing there that makes that "switch user to root" actually happen - and backuppc has insufficient access rights to tar your files
although I could be wrong, I've never seen a sudoers file in my life and am going by what I can understand from the man page.
Apart from that, I agree with the others here: leave those sudo commands out of your script and run the entire script as root (via sudo) or from a root crontab.
Alternatively, give the backuppc account sufficient access rights (eg through groups)