App purchase in App Store can't be completed

I tried to purchase & install a new app for my macbook pro & got a message saying purchase could not be completed and was cancelled. It doesn't tell me why it couldn't be completed. I have tried it for a couple of different apps to see if it was just a problem with that app, but I get the same message with every attempt. I have purchased other apps in the past, and did verify that my payment method info is up-to-date. What is wrong here?