The encrypted-password expects you to put a string that starts with $1$. It is the crypted form of plain-text form.
For instance, I configured the root password by using plain-text and in the config shows like this:
set system root-authentication encrypted-password "$1$tXuEGVir$3t27CvzLoOT5z091bKGX.1"
If you know the password, you can set the encrypted form of it and assign it to another user. Then you will be able to use that password as if you would have set it using plain-text form.
To get a start with Juniper study you can use Fast Track. You have the documentation necessary to pass JNCIS-ENT/SP/SEC.
https://learningportal.juniper.net/juniper/user_fasttrack_home.aspx
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