Line : 13 (Level : 1) This property applies to block-level elements. : div.heading

This is the message that I get re {text-align: center;}
What is it telling me?

quite probably that there is an error further up in the style sheet. That is throwing the validator out.

more to the point; why are you using a div classed as a heading ?

<h1 to 6> tags are for headings and will give the document structure, a <div> is purely a container element and has no semantic meaning.