Thread:Extension talk:ConfirmAccount/Could not create directory "mwstore://accountcreds-backend/accountcreds-public/w/wi/wik"./reply (6)

Usually that type of error is caused by a permission issue. The extension defaults to $wgUploadDirectory. "/accountcreds" as the directory to use. Make sure that php can create that directory and any subdirectories (e.g. Let the php user [usually www-data] own that directory).

Well $wgFileSotre was removed, you can still set it in LocalSettings.php. The more proper way would be to adjust $wgConfirmAccountFSRepos