The WA_Auth_RulePasses() function is defined at line 144 of the WA_SecurityAssist/Helper_PHP.php
If you have placed the code:
<?php require_once( "WA_SecurityAssist/Helper_PHP.php" ); ?>
at line 1 of your checkout_success. php page and are still having the same problem, one of two things is wrong:
1) The change you have made to the checkout_success. php page is not being properly uploaded.
2) The WA_SecurityAssist/Helper_PHP.php does not exist in the location that it is being looked for.
Try deleting the copy of the checkout_success. php on the remote server, then re-uploading.