I know this is an older post, but I’ll post my solution as it is different than the others and none of the solutions here seemed to solve my issue. this solution does not involve adding code to the wp_config file. In order to do this you must have access to the WordPress database.
step 1. locate the WordPress database and the wp_users table.
step 2. locate your user within the table and note the user_ID
step 3. locate the wp_usermeta table. In the table locate the record with your user id and the meta_key ‘wp_user-settings’
step 4. change this value from ‘editor=tinymce’ to ‘editor=html’
step 5. save change, reload admin portal on wordpress site.
You should now be able to edit content on the page.
