In the Google Webmaster Tools you have two options to verify your website. Option 1 is to upload a HTML file and Option 2 is to add a meta tag.
If you created your site using Google Sites, you can verify your Google sites using "add meta tag" option. For this copy the meta tag from Google Webmaster Tools website.
Then open the site you want to verify. In the top right corner, click at Site Settings.
Then click Other Stuff.
In the Google Webmaster Tools verification box, paste the meta tag you copied from Webmaster Tools, and click Save changes.
Now you can go back to Google Webmaster Tools website and click verify.
Please let me know if you face any issue I will respond to comment promptly.