Non-root users haven't enough rights to capture packets. To allow non-root users to do that in BackBox, just opena terminal and run:

• $ sudo dpkg-reconfigure wireshark-common (then enter YES)
• $ sudo usermod -a -G wireshark yourusername
• logout from the BackBox machine and then login again.

The user is now allowed to capture traffic from all the devices the BackBox machine has.