Can't restore or update IPhone - TATSU declined to authorize
for anyone else who comes across this or something similar. It turned out that my hosts file had an entry to gs.apple.com which was causing apple's signing (TATSU) server request to fail with their not so helpful error message. 

So you need to check your hosts file on the computer you are running iTunes on to see if there are any entries to gs.apple.com and then either remove those lines or comment them out by placing a # at the start of the line. 

like so #gs.apple.com 74.454.656.343  easiest way is via terminal. look up how to edit host file if you don't know how. on my mac the host file was located at /etc/hosts  once in terminal: sudo nano /etc/hosts will open an editor, once changed control x will exit, type y then enter when prompted to save your changes to the hosts file. all done.