Hi there,
I got same problem as you I am getting this error
/Users/travis/build/mavlink/qgroundcontrol/src/comm/SerialLink.cc:231 - "open failed "Resource busy" QSerialPort::SerialPortError(PermissionError) "/dev/tty.SLAB_USBtoUART" true"
I tried to reinstall drivers but no luck.
MacOS High Sierra 10.13.6
Thanks for any advice