I purchased a brand new MBP and a cable was included in the box, it has USB-C type connectors on each end and works for charging the computer.
When, instead of charging with the brick, I plug it into my new iMac, I get this error message:
Why do I see this? How do I fix the problem?
This message implies that I have connected the "accessory" (iMac) using a non-Thunderbolt port on the MacBook Pro. However, the marketing materials on apple.com tell me that all the ports are Thunderbolt ports: