Hello Paul,
There is few information so I will assume you have a KMS capable MCU and KMS image was not erase. And right COM port was selected.
In this case, I suppose you have and old image, not supported in KMS1.2 and you need to update it. After communication fail screen, go to Project menu and Select Load Application Image.
Reset board and try to connect again.
Thanks a lot.
Renato Kiss