TATSU ERROR

I've been trying for days to restore/update my iPhone 6s plus using iTunes.

Here's what led me to iTunes updating:

- My iPhone started only loading the apple screen

(The one you see when trying to turn your phone on)

- Instead of loading me to the lock screen (putting your thumb print), the phone began turning into a black screen. In other words, it would go to the start up apple screen and then turn itself off again.

- After reading quite a few people's opinions about what the issue may be, I learned to put my iPhone 6s Plus into recovery mode.

- Instructions further explained to update my mac, iTunes, and then my iPhone

- After doing so, I attempted to update my iPhone

- A reoccurring error populates on both my Apple Desktop computer as well as my Apple Laptop computer (in iTunes)

This is the error:

"The iPhone "iPhone" could not be updated. TATSU declined to authorize this image on this device for this user."

- After trying to research further and determine how to get around or "fix" the TATSU issue, I've come up short. I'm officially stumped and can't get past this error.

Please, anyone, who knows who to fix this- HELP ME!

Things I've already tried:

- Terminal search

- Restarting computers

- Restarting iPhone