Is your internet connection disrupted when Fiddler Everywhere is up and capturing, and is it restored once Fiddler Everywhere capturing is OFF? When Live Traffic capturing is ON, Fiddler Everywhere set itself as a system proxy. You might want to check if that is happening (by looking up the OS network settings for the active adapter) and if another network tooling is not preventing the proper configuration (firewall, antivirus, VPN, etc.).
Here are some KB articles with instructions for troubleshooting on macOS:
I recently download Fiddler everywhere on my Mac and use it to capture authToken of the app on my iphone. However, it always shows that 'No Internet Connection' in both iphone and macbook pro. I configured based on the official website, and wonder what happened? Can any one help me on that?