A couple of things:
Have you tried using -T to specify a target? I don't believe I've ever been able to get anything more than wireshark-style packet sniffing without specifying a target along with the HTTP proxy command. Look at the bettercap website for details.
Is there a reason you're using BackBox? Not that there's really anything wrong with it, but I'm pretty sure you're going to get better support, better compatibility with ruby, and more up-to-date libraries with newer Kali and Debian distros. Don't expect newer tools (especially pen-testing tools) that are updated frequently to work with a distro that hasn't updated their downloadable image in over 6 months. That's a pretty long time in the world of infosec.
As for those wondering about getting it to work on the pineapple: save yourself finding out that the pineapple doesn't run it well and just get a RasPi 3 with Kali. My mobile setup is a Nano with a AWUS Alfa 036NH added to it, RasPi 3 model B configured to auto-connect to the MGMT AP on the Pineapple on boot, running Kali with Bettercap. I control the Nano via webui on my iPhone, and the RasPi 3 via vSSH lite (free SSH), all battery powered. The alfa card is used for the mgmt AP, and the range is fantastic. With some practice you can do a ton of really cool stuff with it.