Meaning of AMD-Action:authenticate:SP
This is most likely a temporary glitch at Apple's end. 

I'm aware of a number of users who have received the exact same error message during the past few hours while trying to authenticate (i.e. login) with the Mac App Store. In all cases, they were just trying to install updates (not upgrade macOS).

The error basically means that Apple's servers can't authenticate your account. I'm not sure if location is an issue, but in my case the users are based in Australia.

I imagine it'll be resolved reasonably quickly, so I would wait and try again tomorrow.