Now I'm stuck with TWRP and no matter what I do, I cannot figure out how to root. I've flashed 3 different versions of Supersu from recovery and none of the zips I install offer me the option to root. I've tried to flash Supersu through adb sideload. I've even tried using ROM manager to reboot and install a copy of Supersu downloaded off my phone's Chrome (probably didn't work because I'm not rooted - it tells me "an error occurred while trying to run privileged commands").
I was under the impression that after installing the Supersu zip from recovery, you would be asked to root either directly after installation or upon rebooting. However, when I reboot, TWRP warns me that "There is no OS installed, are you sure you wish to reboot?" From what I've read, that normally indicates a soft-brick, but my phone is definitely not bricked (yet).
How can I flash Supersu in a way that gives me root?
I'm fairly competent with adb, but nowhere on the internet have I found a problem even remotely similar to mine....