Joonla 5 has the good idea of enabling some quite flexible password format restrictions to be set. However the bad part is that they have set some stupid default.

In particular when installing J5 there is a requirement for the first admin user which you define during installation to have a minimum length of 12 (yes twelve) characters. In order to change this you need to edit a file before you start the installation.

The simplest thing is just to edit /installation/forms/setup.xml and set the field admin_password | validation="" This will enable you to use whatever you like for a password - it's your responsibility to make it as secure as you want it to be.

While you are about it you might like to also edit /administrator/components/com_users/config.xml field password_options | minimum_length | min="6" default="8" to what ever value you want to enforce for all users.