With LSAT practice tests you can be sure the test is never wrong. The rules never contradict themselves in a manner that'll make it impossible to answer a question. It is therefore far more likely you've misunderstood a rule. I just did this problem yesterday.
Question fifteen introduces block Y _ _ Z
in order for this condition to hold you must have the following order
T Y W S V Z X
E: is correct by default at this point, but we don't know what W is according to the diagram so obviously this is the answer.