Using mtr installed via Homebrew on my Mac I received the above error.
However, I tried answering this on question on serverfault which has (at the time of this writing) 455 views, but this wasn’t possible as it was closed (off-topic), so I’ll answer it here due to the high interest:
Preface the command with
sudo in addition to specifying the
sudo mtr -6 2001:db8:1d4f:10::1
That clears the error and mtr now functions as expected.