I see, sorry did not realize this was wordpress.com.
A quick search states iframes are not compatible with wordpress.com.
“No javascript or iframe embeds from unsupported sources or third party advertising codes can be used on any wordpress.COM hosted sites.”
https://wordpress.com/forums/topic/embeding-an-iframe-code/
Someone suggests installing the iFrame plugin in wordpress.com to try to solve the issue but not sure if that works. Otherwise its only possible on wordpress.org