I tried before one hour ago with Raspberry Pi2 and I achieved to configure Motors.
Now I have a problem to connect my ErgoJr via Snap what should I write as a host name?
Sorry I did not read that you where using a raspberry pi 3 before. Indeed, they change the way they handle serial communication. In order to work with a Pi3, you have to:
add enable_uart=1 in /boot/config.txt
uses /tty/serial0 instead of /tty/ACM0 in the configuration file of your robot poppy_ergo_jr.json
It depends on the hostname you used when configuring the robot. By default its poppy.local.