Hi: I have just installed Debian Stretch on an Acer Aspire One Cloudbook, The installer gave me the option to let root to login. I chose it and wrote the password. But when, after the install was finished, I wanted to login as root, the password was rejected. Even a su - as regular user does not recognize the password. What am I to do?

A simple web search returned a lot of how-to articles. Did you try any ?
Take a look here & here

That's neat! I would probably have booted the installation disc or SystemRescue and corrected the password from there, but booting the installed system straight into bash is simpler.

I'm curious though to know what happened to the original password. You always have to give a password twice when setting it, so it's not likely to be a simple spelling error.

The installer most likely prompted you to create a root password, as said above, enter it twice to confirm. Are you trying to login to a GUI in Debian? From what I understand (not being a Debian user) that isn't allowed in a default install and a user needs to set it up after the install. Clarify the situation please.

https://economictheoryblog.com/2017/...n-in-debian-9/

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probably a keyboard layout problem. I guess.

It's simple. Just after the install I booted and then I am in the GUI. If you want a text console (/dev/ttyN) in Debian Stretch you must press Ctrl-Alt-<function key>. Here if I want to login as root I can't. If I go to a text console and login as root, the password is rejected. Typing su - and giving root's password has the same result. In the GUI, opening a terminal an typing su - is just the same. So, no way to do anything as root, for the sudo command is not installed and, if it were, it needs root's intervention to be operational.

These days, most distros will not allow you to log in from the display manager, that is, log into the GUI environment, as root.

It's been several months since I installed Debian, but, as I recall, after the root password is created, it prompts you to create a user.for creation of a user.

did you try to reset that password already?

Yes, I boot with init=/bin/bash and the other thing (i do not remember now). Then changed the password (passwd command). As a result, exactly the same behavior. Thinking it had to do with keyboard configuration, as I had skipped, when installing Stretch, the keyboard configuration, I reinstalled and went over the keyboard configuration, choosing the first item (something like american keyboard). My machine happens to be an American one, with the classical keyboard keys distribution. I even altered the password, which made use of a special character, and changed it to a number. But still same behavior.

I think it may be a bug in the installer, and I intend to report it. Perhaps using another version of the same installer, either an older or a newer one, the problem could be solved.

I don't think, but probably....
I would first try a simple password, which contains no tricky key, like ppaa22wwqqgg