On one of my sites, having updated to WordPress 5.0 and its new Gutenberg editor, I found I couldn’t update a new or changed post. Instead, an “Updating failed” message appeared.
To remedy this, I installed Classic Editor and Classic Editor Addon to replace Gutenberg. In other words, back to how it was in 4.9.
(The Classic Editor Addon prevents accidental activation of Gutenberg.)
Both plugins are available from WordPress. Select Add New from the Plugins menu and enter “Classic” for the Keyword search.