I'm signed in to iCloud, iTunes and whatever but it won't let me sign in to the Mac App Store. When I click the sign in button on the bottom left a window pops up with my Apple ID filled in and I put the password in the bottom field. When I put the correct password in, click "Sign In" the pop up goes away and nothing happens. For troubleshooting purposes I tried an incorrect password and I get "Your Apple ID or password is incorrect." If I try to purchase something (A free item) I get the message "An error occurred during purchase. The operation couldn’t be completed. (com.apple.commerce.client error 500.)" Your only option is to click "OK" What I've done to try and fix this. Signed in an out of iTunes just to see if there is a problem with my Apple ID. I deleted the "com.apple.appstore.plist." rebooted, and I get the welcome to the App Store pop up but the same problem. I'm on a 2018 Mac mini that's about a week old, has no software or apps installed other than what came from the factory. I take that back I did Software Update so it's currently running the current version 10.14.3. Also FYI in case I'm overlooking something obvious this is my first Mac in about ten years.