Mac App Store Download Failure - Clear Cache?
Got this sorted out.

Quit App Store.

Use Terminal or Activity Monitor to quit/ force quit  the following processes and to delete the following folders:
 killall storeaccountd
 killall storeassetd
 killall storedownloadd
 killall storeinappd
 killall appstoreagent
 rm -rf ~/Library/Caches/storeaccountd
 rm -rf ~/Library/Caches/storeassetd
 rm -rf ~/Library/Caches/storedownloadd
 rm -rf ~/Library/Caches/storeinappd


Restart App Store and hold the option key down to cancel any stuck downloads.

Then try a download again