Sorry for all the confusion, yes you miss a lot of features if you don't utilize the Cornerstone for building your pages.
From the screenshot you showed above, you should choose a page that you want to edit in Cornerstone. But that page needs to be built in Cornerstone in the first place when you created it. Do not edit pages that are not originally built in Cornerstone, you will lose the content of that page.
More details of that here (Cornerstone - Introduction)
Now back to the original issue, please provide us the direct page URL and point us the Headline that you're having an issue changing the letter-spacing, so we can see what is happening in there and provide an alternative solution.
Have a nice day,