"But not Chris" is a sentence fragment. In real life, we use fragments in our writing to make it more interesting, to add variety to a paragraph, and to avoid repeating unnecessary information.
In this case, "a blank wall" is a metaphor. His goal -- unemployment benefits -- are on the other side, and there are no doors or windows for him to pass through. It also means that the person was unwilling to help him.
Student or Learner