I need the authentication code to install adobe flash

When you start an administrative action from a terminal windows by giving a "sudo ..." command, you get a request to enter your password as confirmation for that administrative action.
You have to be aware that you do not get any feedback when typing the password in this mode (no echo, not even '*' placeholders).
Just "blind" type the password and press the "enter" key.

the problem is that i forgot the password , what can i do

If you do not know your own password any more, there is a procedure described in http://www.psychocats.net/ubuntu/resetpassword

 I already tried to do it but when I try to type the new password, nothing writes I do not know why

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      De : Manfred Hampl <email address hidden>
 À : <email address hidden>
 Envoyé le : Samedi 31 mars 2018 20h46
 Objet : Re: [Question #667306]: I need the authentication code to install adobe flash

Your question #667306 on Aptdaemon changed:
https://answers.launchpad.net/aptdaemon/+question/667306

Manfred Hampl proposed the following answer:
If you do not know your own password any more, there is a procedure
described in http://www.psychocats.net/ubuntu/resetpassword

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When you type a password in command line mode, you do not see what you type.