Mac Serial ERROR 9

Trying to connect an FTDI USB-Serial adapter to a Macbook 11,1 running Catalina, we’re getting the following error code:


Can anyone please tell me what this error code means? It’s not in the Xojo manual, and doesn’t help. Thanks.

from MacErrors.h:

dsLineAErr = 9, /line 1010 trap error/


#define EBADF 9 /* Bad file descriptor */

So maybe a the second?