app store errors

Update 02-05-18: To avoid confusion for anyone looking up this topic: The title of this thread might now be a little misleading. I tried to make my first CustoMac on a HP Compaq Elite 8300 SFF... First I tried High Sierra, which I could not fully download. Hence the name of the thread. The...

I am able to sync my notes & calendars through iCloud and I have my Apple ID associated with my user account, but I cannot use iTunes or the App Store. When I try to buy/install an app from the App Store, I get small red text under my Apple ID that says "An unknown error has occurred". Clicking...