A lot of people are having issues with the new Gutenberg editor. To bypass it, you can either revert back to Wordpress 4.9x by re-installing those core files by FTP. Or go to Plugins > Add New and by default the Classic Editor plugin should appear, if not search for it and install it and activate it. It will give you back your original WYSIWYG editor and there are some page builder plugins that break with Gutenberg and the Classic Editor should restore them.