WordPress 5.0 was released in december 2018, and with it came a new editing experience known as Gutenberg.
Not everyone is happy with this new forced update!

Gutenberg is an upcoming WordPress editor with a goal to modernize the WordPress writing experience.
The idea is great, but it is not yet working for all users. Especially if you are using a different editor, an editor that came with a theme or are using certain plugins. In this case you might want to disable the new editor…..

HOW TO DISABLE THE NEW GUTENBERG EDITOR?
Just install the ‘Classic Editor’ plugin. Go to Plugins and click on Add new plugin. Search for Classic Editor, and click on Install now, after the installation is completed, all you need to do is click on activate and you’re good to go!

Once installed and activated, the Classic Editor plugin will basically supersede the new editor, Gutenberg, and you’ll be able to add and edit your posts just the way you always did.