If you ran across my previous post about disabling the Proxmox no subscription pop-up, you might also be wondering why you get an alert on the console regarding the scheduled update job. The reason that this shows up is that the apt-get update job returns an error code for one of the Proxmox Enterprise repos.
The fix is relatively simple, just reconfigure your sources.list.d contents to not have the pve-enterprise repo, but instead to have the pve-no-subscription repos enabled instead. The wiki has a nice article on the various package repos used for Proxmox.
Please note that the entry for the pve-no-subscription repo merges the sources.list and the sources.list.d file into one.