It means what it says: You need to save as IDML from the newer versions of ID to be able to open it in older versions. there is no magic wave of the wand that can fix it.
Moving this discussion to the InDesign forum.
CS6 was the first version of InDesign that was available through a Creative Cloud subscription, and it is also still available by perpetual license. Creative cloud is now up to version CC 2014 (version 10), which is tow versions newer than CS6 (version 8). You can install CS6 on the new machine as part of the subscription, or if the CS6 is part of a subscription plan on the old machine you may be able to install the newer version there, depending on the OS.
You write: “the computer came with the old CC programs which I try to update to CS6”.
CC is newer than CS6, not older