uploading Error "/dev/cu.usbmodem1421" mac os sierra - arduino uno

Hi everyone,

I can't upload anything on my arduino UNO from my macbook with os sierra. The board works fine with windows though. From my macbook I keep getting the following error.

avrdude: ser_open(): can't open device "/dev/cu.usbmodem1421": No such file or directory
Problem uploading to board. See http://www.arduino.cc/en/Guide/Troubleshooting#upload for suggestions.

I found many posts about similar issues but no clear solution.
Anyone out there with an idea of how to solve that?

Thanks in advance