I installed Fiddler on a new Windows 10 workstation.  When starting up Fiddler, I get the following error message: “Failed to register Fiddler as the system proxy.”

When I try to capture traffic (F12) I get the same error message. No
traffic is captured.

Netstat shows Fiddler is listening:

C:\>netstat -an | find "8888"
TCP 127.0.0.1:8888 0.0.0.0:0 LISTENING

After searching high and low, the fix is quite simple.  Just go and start the WinHTTP Web Proxy Auto-Discovery Service.  It was disabled on my workstation.  After that, Fiddler works as expected.