Wordpress 5.0 has arrived and it got a whole new way for you to do the editing, using the block style. The Freaking useless Guternberg, which the developer claim will be loved by blogger/editors..
If after your side updated to WordPress 5.0 (and now 5.0.1 to fix the vulnerable issue), and found out that you are unable to save draft or publish your new post, even the Tags and Categories goes haywire.. Well, do try to install either 1 of this 2 plugins, to temporary solve the issue, which is to make the latest editor mode disappear.
After install and activate the plugin, and yup, you are good to go and continue editing as normal in WordPress.