Please post the link here, we can’t help you without the link to your page, we have to test it in the validator too to help find the problem. Also, sometimes there might be some timeout depending on network traffic between our servers and yours, try again after waiting 10-15 minutes just to be sure.
Adding a robots.txt is not the way to solve most issues (except when your robots.txt disallows Twitterbot to crawl your pages).