After the initial startup, the Fiddler Everywhere application could only capture non-secure traffic (HTTP) while SSL traffic is not captured. To enable capturing and decrypting HTTPS traffic, you will need to explicitly install a root trust certificate via the HTTPS sub-menu in Settings.
The button installs and trusts the Fiddler root certificate (macOS and Windows only).
The option defines if Fiddler Everywhere should capture HTTPS traffic or skip it. By default, this option is disabled. To enable the HTTPS traffic capturing, first you need to confirm whether the root certificate is trusted. If it is trusted you can simply enable the capture option.
Expand Advanced Settings drop-down to show the Export Root Certificate to Desktop and Trust Certificate button. Click the button to export Fiddler the root certificate to the Desktop folder for manual import and trusting of the certificate.