I have some links for you; hopefully one will work with your version of OS X:
https://www.mac-usb-serial.com/wordpress/support/troubleshooting/
https://www.mac-usb-serial.com/wordpress/support/de-installation/
and for SIERRA only:
https://www.simplicate.info/2016/09/16/ch340-drama-osx-10-12-sierra-16a313a/
and latest driver:
http://www.wch.cn/download/CH341SER_MAC_ZIP.html