New MBP (touch bar) won't boot from external HD

We just received a new MacBook Pro (15-inch, 2016) with the touch bar. I'm trying to boot up from an external hard drive that has our Mac image installed on it that's running El Capitan. When I hold down the alt/option key, it displays the HD as an option, which I can select, but then it returns a black screen with a white circle that has a line through it, supposedly the "prohibitory symbol" which means it can't find a valid system folder to start up from. However, our other new MBPs without the touch bar (Retina, 15-inch, mid-2015) can boot up just fine from it.

The external hard drive (Seagate) has a USB 3.0 connection. I'm using an Apple-branded USB to USB-C adapter to connect it to the Mac. I'm guessing the adapter might be causing a problem, but can't find any info on this.