I cannot sign into the App store. Any time I attempt to I get the same error
I am sorry to tell you, that is normal.

In times of new product release (OSX Mojave), they servers just get flooded with requests.

You are connected to App Store, but ca not pass the Log-In server.

It says, an Unexpected Error ?, 

if anything is wrong with you profile or your log-in information it would tell you that.

The problem is not on your side !

Just have to keep trying.