Videos can't play in this copy yet

You're opening MarketTube as a file (or a preview), not as a website. YouTube's player refuses to run there and shows a "configuration" error, so the video can't play inside the page.

Put the file online once and every thumbnail plays right in the page, for anyone, with no sign-in:

1. github.com → New repository → name it markettube → Public
2. uploading an existing file → drag in this index.html → Commit
3. Settings → Pages → branch main → Save
4. Open https://YOUR-NAME.github.io/markettube/