Accepting payments for your events with Event Calendar App requires that your site is using a https connection and is secure.
You can tell if this is so by browsing to your website and looking for a padlock symbol in the URL bar.
A secure site will look like this:
Notice the padlock.
A nonsecure site will look like this:
It's highly recommended, irrelevant of Event Calendar App, that your site is secure. In the future, Chrome in particular will display warnings for all websites that are not HTTPS.
How do I make my website secure?
This completely depends on how your website is setup. Contact your hosting provider or the person who made your website for you, they should be able to help. The process shouldn't cost you much, or anything at all. SSL certificates (the thing that make a website secure) can now be obtained for free.