I think the problem is probably just that "1" isn't a valid number of clusters (though it does actually seem to accept "1" after clicking OK). Try "2" instead - I think that should solve it. The following works for me:
FakeData = NewTable("New Cluster Summary",
New Column("Week1", Numeric, Values([1, 4, 6, 8, 2, 3, 5, 7])),
New Column("Week2", Numeric, Values([3, 1, 8, 5, 4, 9, 0, 2])),
New Column("AttributeX", Numeric, Values([1, 1, 1, 1, 2, 2, 2, 2])),
);
FakeData << K Means Cluster( Y( :Week1, :Week2, ), By(AttributeX), Number of clusters(2), Go );