Can't connect printer to repetier-server on RPI3
Thanks to the repetier team on git hub, we found that the problem came from a permission issue on /dev/ttyUSB0, as the user repetierserver couldn't access it.

Other people reported that these commands solves the problem:
sudo adduser repetierserver dialout
sudo adduser repetierserver tty
but in my case it seems I need to run :
sudo chmod 777 /dev/ttyUSB0
in order to solve this.