Errors flashing on macOS

Hi @zachrattner.

It is a "hallucination". Although other boards do go into a special mode after a double reset, the Nano Every does not. So this doesn't accomplish anything.

If you are looking for a way to put the board into the special mode used for uploads, this is done by momentarily opening the serial port at 1200 baud. You can even use the Arduino IDE Serial Monitor to do that, as described here:

That issue is about the same error you are experiencing. However, the cause of the failure reported there was understood to be specific to Linux, so I'm not sure the error you are experiencing has the same cause (a given upload error message may have multiple completely distinct causes).