At some point during the EFCT16 I also ran into this. I wrote a post on authorising from within TWRP - though this will likely only work if the phone isn’t encrypted. It could be that if your device is encrypted that the file with the authorisation signatures isn’t readable when in TWRP, causing no authorisations to be available. Sorry if seems a stupid explanation - I’m not that aware of how this whole Android thing is designed to work.