Twitch uses RTMP for streaming so it’s important to properly configure it.
Please login to your Twitch account, go to Dashboard and find the Stream Key. Then please create new RTMP server in ManyCam:
Name: choose the name you like (e.g. twitch)
Stream key: the code from your Dashboard panel
You don’t need to use “Access required” (so it must be disabled in ManyCam RTMP server settings) and your login/password for twitch (this option is available in ManyCam 5.4). Then please start streaming and check if you can see video preview on Live tab in your Twitch Dashboard.