A quick WordPress helper tutorial for you, as we’ve been asked this in a couple of different ways over email of late, by people who had experienced PHP errors in their WP installations.
PHP Fatal error: Call to undefined function request_filesystem_credentials() in [somelocation]/wp-admin/includes/class-wp-upgrader-skin.php on line 93
How to resolve this error
This issue is most likely caused by an attempted automatic update from WordPress version 4.9.3 to the (current) version 4.9.4. There was an issue with version 4.9.3 which broke the automatic update feature in WordPress.
The solution is to “manually” upgrade WordPress, either by clicking on “update” or, if this does not work, downloading and updating over FTP… or command line. Whichever method suits you best. But you need to update to 4.9.4 (or higher) as the 4.9.3 version can. Not. Automatically update.