HTTPS stands for Hypertext Transfer Protocol Secure. It is the protocol where encrypted HTTP data is transferred over a secure connection. By using secure connection such as Transport Layer Security (TLS) or Secure Sockets Layer (SSL), the privacy and integrity of data are maintained and authentication of websites is also validated.
For this reason, we only accept campaign URLs served over HTTPS. Chrome, for example, won't display a non HTTPS website in an iframe of a HTTPS website. It gets blocked unfortunately.
In order to use the HTTPS version of your website, it is required that you have an SSL certificate. If you don't have it yet, you can register with CloudFlare (https://www.cloudflare.com/) for example and access their free plan.
Or if you are promoting an affiliate campaign, you can contact your affiliate network to obtain it.