Fix: xcrun: error: invalid active developer path (/Library/Developer/CommandLineTools), missing xcrun at: /Library/Developer/CommandLineTools/usr/bin/xcrun
The problem is that after I have upgraded my Macbook to the Catalina, I lost the CommandLineTools. In my case, I have installed it in a new partition. It was a clean partition with nothing in it.
It was an eye opening incident for me to understand that not everything gets shipped with the OS.
You only get the tools when you ask for them. Not by default for any layman user out there.