Have you flashed anything else? I'm not very familiar with TWRP yet (*), but recoveries on my previous device generally had an option to toggle signature validation, and if that was accidentally set on you couldn't flash unsigned zips (i.e. almost everything you ever wanted to flash!). This caught a few people out, so I suggest looking for this just in case it's relevant here.
(*) And can't just boot into it to test because I actually only did the temporary install so far. Reason being that we're expecting an OTA soon, and I thought I might try to update to that (and capture it as a backup) before I started playing with ROMs.