It might be helpful if you posted a screenshot of the device at the point of the prompt, but unfortunately this isn't something related to Fiddler itself so I'm not sure how much help I will be.
It's possible that whatever version of Android you're using requires a password of a higher complexity after you install a certificate? Or that it's asking you for the device's existing password? I can't really say.
I can say that Fiddler's certificate itself (e.g. the .CER file downloaded from http://fiddlerPCIP:8888/FiddlerRoot.cer) does not have any password protection.
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