You may want to refer to the embedding documentation
Rather than a straight iframe
For a straight iframe you want, which is the same as what you currently have
<iframe
src="https://player.twitch.tv/?<channel, video, or collection>"
height="<height>"
width="<width>"
frameborder="<frameborder>"
scrolling="<scrolling>"
allowfullscreen="<allowfullscreen>">
</iframe>
I don’t see anything wrong with your page other than you are not passing a username to the iframe so there is no channel to load.
You actually need to pass in the channel name to https://player.twitch.tv/?channel= so it’s not jsut https://player.twitch.tv/?channel= but https://player.twitch.tv/?channel=esl_dota2 for example