Adding noarchive to robots.org

A meta tag goes into the HEAD section of the HTML of your pages, not the external robots.txt file. So the task is finding a way to inject this meta tag into all pages on your website.

You could edit your theme’s header.php and add the code if you wanted to, but the easiest way (which also guarantees that the code will remain in place even when you change themes) is to use a simple plugin like “Insert Headers and Footers”

https://wordpress.org/plugins/insert-headers-and-footers/