Mac App Store Won't Download Updates

I have a Late 2015 iMac 27-inch Retina 5K with 32GB of Crucial RAM in it. It is running macOS Mojave 10.14.6 with all of the latest system updates to this very date of posting. The internal 1TB mechanical hard drive was so unbelievably and ridiculously slow (thanks to APFS) that I bought a Samsung T5 1T external SSD. I formatted the new SSD for APFS and used the latest version of Carbon Copy Cloner 5.1.21 to transfer the “clone” of my internal hard drive to the external SSD.

My iMac starts up perfectly with the new SSD, 3 to 4 times faster than it ever did before and everything works perfectly. I immediately unmount the old internal mechanical startup drive after my iMac boots up so that it is nowhere in sight and does not get in the way.

I've been using Macs for 25 years so I like to think that I know what I'm doing. But -- there is one annoying problem that I have that I just simply cannot solve on my own, even after looking at numerous websites and forums on the Internet.

The Mac App Store shows that I have six software updates, one of which is GarageBand 10.3.5 which I need. When I click the update button for any one of these six updates, the circular progress wheel spins for about three or four seconds and then just stops. Nothing downloads at all. 

I have deleted the App Store preference files, cache files and also the following files as recommended on many sites:

sudo rm -R $TMPDIR../C/com.apple.appstore/*

I have logged in and out of my account, I have logged in and out of the Mac App Store. I have restarted my Mac numerous times. I have killed the softwareupdated process in Activity Monitor. I think I've tried absolutely everything that I could find on the Internet. Nothing I’ve done solves the problem.

What is happening?