It seems there is a bug in vCenter 6.7 which caused the update check of VCSA is not working.
So to upgrade VCSA from 6.7 to 6.7u1, you need to
1. In the vCenter Server Appliance Management Interface, go to Update > Settings and configure the custom URL to https://vapp-updates.vmware.com/vai-catalog/valm/vmw/8d167796-34d5-4899-be0a-6daade4005a3/126.96.36.19900.latest/.
2.Re-try the upgrade.