By default, the WordPress WebARX plugin injects certain security headers into the server response of your site. However, these security headers prevent MarketPlan.io from working properly on your site. This most likely happens because they insert your site as an iframe into their application which the X-Frame-Options security header prevents from doing because it prevents clickjacking attacks.
In order to make it work, disable the security headers setting by going to to admin page of your WordPress site > Settings > Security > Firewall menu page > Uncheck "Add security headers" > scroll down and click on the save settings button.
It might take a few minutes before the security headers have been removed from your site.