apt is a high end package manager, meaning it resolves dependencies, fetches them, and all that fancy schmizzle, Dpkg, is a low end package manager you will have installed, which apt utilizes to do the actual installing of .deb packages.
rpm is a low end package manager, very similar to dpkg, however it is designed to work with .rpm packages instead of .deb packages. I'm assuming you're using Lubuntu. Lubuntu is a .deb based distro, which means, it should not have to deal with rpm packages. I have no idea why you have the rpm package manager installed when you have dpkg. Runnning,
sudo apt-get remove rpm
should fix your issue by simply removing rpm.
If you desire to install a program, do it with, 'sudo apt-get install "foo"'. If you need to do a manual install by downloading the package off the internet, and then installing later, then use dpkg, not rpm, which means you download the .deb package, not the .rpm package: always try sudo apt-get, before this method though.