Your code is correct
tested it at
https://twitch.extensions.barrycarlyon.co.uk/temp/test_23884.html
If it’s not working for you, check the console for any errors. but this is a straight forward html iframe so there shouldn’t be any issues, even if loading the main page over http instead of https.
The use of a website builder shouldn’t make any difference. But wix I know can be fiddly at times. They might have a specific guide for using iframes.