Earlier quoted context omitted.
Why can't debugging be tested?
I can think of a couple challenges. Let's say you're debugging "the website won't load." Often debugging is a hunch, and quickly branches. One person will go down the path of trying the same site from another PC. Another will try a different site from the same browser/PC. Another will try to ping the server. If you're running this live you're always going to have the chance where someone's hunch or first try basicall…
E.g. "I am going to ping an IP on the outside first, to see if ICMP message reaches the outside server. If yes I will check if the DNS server responds, if no we check the physical connection."
That would be an ideal answer, judging less ideal answers fairly is certainly a challenge, but not impossible.