This cat scratching post features arches that may appear to be missing from your package.
The scratching post is designed so that parts "S" and "T" are actually boxes constructed from parts "M", "L", "B" and "P", "Q". The arches are formed when the material parts are set up.
Please see the first three steps in the manual here for details on assembly.
Here are a couple of images detailing the material component for parts "S" and "T" before and after assembly: