I am currently using Wordpress version 5.0.2, i want to add news posts but the button "ADD BLOCK" is not active.
In my case this was caused by WordPress requiring
User-Agent header present, which AWS CloudFront CDN by default do not pass (and in most cases due to performance reasons, should not).
My workaround for this, as described in AWS CloudFront Configuration for WordPress/WooCommerce - About Lovia:
User-Agentheader to be passed by CloudFront in order to enable the visual editor. There are several ways to work-around this, but our best practice is to add additional behavior for path pattern
Note: "Disable the visual editor when writing" must be unchecked.
Additional info: https://wordpress.stackexchange.com/a/325234/184905.