Help me pls.I ve forgot backup password

When you entered the backup password for the first time (the only time you will ever enter it, except to restore a backup) if you have a Mac and have Keychain enabled you were given the option of saving the password in Keychain (this is the default). So check Keychain for it.

If you haven’t erased the phone yet you can create a new backup with a new password—>About encrypted backups on your iPhone, iPad, or iPod touch - Apple Support

If neither of those work you get unlimited guesses, so try to guess what password you might have entered back when. If you are like most people it will be a password that you reused. And to speed up the process click to change the password and you will be prompted immediately.

if you forgot the encrypted backup password, you cannot retrieve it..

I believe you can try the password as many times as you'd like, so try your commonly used passwords?