<p>The error is that the <code>array</code> environment has a required argument that gives information about how the columns should be aligned, and in your case, the initial <code>\mathbb</code> is read as that argument.  The characters that are allowed for alignments are <code>c</code>, <code>r</code>, and <code>l</code> (for center, right, and left), plus <code>|</code> and <code>:</code> for vertical lines and dots.  Since none of the characters in <code>\mathbb</code> are valid, they are ignored.  But the effect is that the <code>\mathbb</code> is eaten by the <code>array</code> environment, and so it is not applied to the P.</p>
