Oracle Enterprise Manager (EM) 13c has the ability to connect to My Oracle Support (MOS) in order to download patch recommendations, information about new plugins and versions. This is pretty useful in terms of keeping your EM up to date.
There was an issue early this year that prevented the connection from EM to the MOS portal. This has been addressed and a patch is needed in your EM in order to restore this functionality.
Take a look at patch 31233849 and MOS note 2697356.1. This patch is already included on the EM 13c R6 update. My recommendation is to apply the latest RU to your EM 13.4 installation.