Why mac App Store downloads won't start?

My Mac's App Store downloads won't start. I have tried signing in & out, restarting the computer and yet it won't download anything. Once the "get" button is clicked, it stays spinning forever as shown in the following screenshot:

My internet connection is good enough to browse the web and watch videos, etc... I am using a slightly old version of MacOS Mojave (10.14.5) in a MacBook Pro (13-inch, 2017). However, that shouldn't be a problem.