I followed this instruction and checked DO_NOT_TRUST_FiddlerRoot is always trusted. But I still can't "Trust root certificate" and checkbox of "Capture HTTPS traffic" can't be checked.
Also I checked key chain and DO_NOT_TRUST_FiddlerRoot status is good.
It looks like that the Fiddler application was unable to verify that the certificate is trusted. Could you please send us the full Fiddler log files so we could investigate what went wrong on your side?
See attachments for log files and let me know if you need anything else.
It looks like that the file that lists the trust certificates is containing an invalid entry (probably an entry that contains a null symbol in the certificate name). You could investigate that file via the following command:
security trust-settings-export /tmp/trustSettings.xml
The above command will export the list of trusted certificates from the keychain application. This is the list used from Fiddler Everywhere to verify that the Fiddler root certificate is installed and trusted. Note that this file contains sensitive data, so do not share it publicly and use it only for local debugging and testing. You could remove the certificate that has the problematic name and then retry the Fiddler trust flow.