The parent should be a domain only. Not a domain+path
<iframe src="https://www.twitch.tv/channel&parent=localhost&parent=share.streamlit.io/user_name/project_name/main.py "></iframe>
should be, additionally your embed URL is invalid:
<iframe src="https://player.twitch.tv/?channel=twitch&parent=share.streamlit.io "></iframe>
Swap channel=twitch for the target channel channel=monstercat