How do I get a variable to dislay hello world on the home page when a subpage is loaded?

I am creating a website using google sites.The website consists of a main page and a couple of subpages.
I want the code to display ‘hello world’ on the main page when a specific subpage is loaded.
I have embedded the code bellow on the main page.
google shows the error message google.sites.com refused to connect

Thank you in advance. Any tip or advice helps.the code i entered