get_query_var throwing a 403 i believe

I’m trying the following in my code and it seems to be throwing a 403, so I would suggest there is something obvious I have not setup – apologies for the newby question.

I am trying to see if the URL Parameter “ThisCancelled” exists and if so show the message in the echo on the page.

PHP Everywhere box reads:

<?php
if (get_query_var(‘ThisCancelled’)) {
echo (“Setup Transaction Cancelled, please try again”);}
?>