you have applied universal email to the thank you page using the any form post trigger.
There is no POST data that will be sent to the thank you page.
the membershipapplication.php page is set to POST to itself, store the information to the database using the insert record behavior, then redirect to the thankyou.php page.
the redirect to the thankyou.php page will send the post data.
you need to edit the insert record behavior and clear the redirect.
then apply the universal email behavior to the membershipapplication.php page, set the go to page to the thankyou.php page so it will redirect after the email sent.