Have figured out the issue with the email template. Your HTML code should look something like this =>
<html>
<head>
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
</head>
<body>
<p>Dear #CUSTOMER#,</p>
<p>Thank you very much for contacting us. The ticket #TICKETID# 's status has been changed to #STATUS# and we would like to get your feedback on our customer care operations in order to serve you better in future. </p>
<p>Please provide your valuable feedback by clicking on this link: </p>
<a href="#SURVEY51#">Please complete this survey</a>
<p>Thank you,</p>
<p>Customer Care</p>
</body>
</html>
NOTE => Please change <a href="#SURVEY51#"> to <a href="#SURVEYXX#"> , where XX is your survey ID. Use the same in mapping too.
Happy surveying!