You have validation on other fields that might be causing it to fail as well.
To determine the exact field write this to the page:
<?php
if (isset($_SESSION['WAVT_sitesndetailsinsert_Errors'])) echo($_SESSION['WAVT_sitesndetailsinsert_Errors']);
?>
That will tell you the error number(s) when validation fails and help you narrow down which field caused the error.