The code is in the wrong spot.
1) Move all of the code above the: <!doctype html>
2) replace this code:
if(intval($responseKeys["success"]) !== 1) {
echo '<br><h5>Please check the captcha</h5>';
} else {
with this:
if(intval($responseKeys["success"]) === 1) {
Then use this in the div where you want the message to appear:
<?php
if(isset($_POST["Registration_submit"])){
echo '<br><h5>Please check the captcha</h5>';
}
?>