I know this post is OLD but if you haven't managed to Flash try updating your TWRP. I ran into the same problem when trying to Flash to my daughters tablet just after flashing it myself on my own . Same model just different TWRP versionsWow. I've used this tablet very little in recent years, but thrilled to see this update!
I was able to flash LOS 14.1. Still struggling to flash this or figure out what I'm missing, but when I do I will buy you a coffee!
Amazing that you did this for a device that came out nearly 7 years ago!