Are you saying that you have a single host with essentials and vCenter server was previously installed on a Windows VM but is now the VCSA appliance?
I'm also assuming that you previously had Veeam installed on the vCenter server, it that correct?
If so you can still backup to a USB disk but you would need to attach the USB drive to a Windows VM, preferably the one that has Veeam on it, once that's done you should be able to setup a backup job and target the USB drive.