Originally Posted by Hadron
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.
if you do get an OTA,you can also capture the ota file in your phone. just navigate to your downloads folder,and copy it to somewhere else(leave the original in place,obviously). it contains firmware that can be useful to have,and we can upload it in case other folks need it
also make sure you havent deleted any bloatware,and do not attempt to OTA if youve changed CID or MID