You've got both an animation in the 3D Viewport set up, as well as a Sequence in the Video Sequence Editor within the same scene. The sequence in the VSE now seems to take precedence over the 3D Viewport Rendering. So essentially, when you do a render, instead of using Blender Render to knock out the frames of your animation, the VSE is trying to read the PNGs defined in the sequence strip, and spits out those.
To "fix" this, you can disable the Sequencer output in the
Post Processing tab under the Render Settings. Disable the checkbox called
Sequencer and render your animation.
Once done, you can enable the
Sequencer checkbox again when you're ready to cut the video strips.
Personal Recommendation: To avoid this kind of confusion, create a second scene in your Blend File to separate animation from video editing. This way, Rendering and Animation can have their needed settings, while Video Editing can have completely different ones.