Include a link to the source code repository of your project, because we really need the context of seeing your templates and partials to be able to help you. It is trivial to do a quick git clone on your repo, then run hugo server in your project, to help you out. On the other hand, recreating your code from screenshots, or sort of guessing at it, is not.
If you can’t share your repository for whatever reason, consider creating a dummy repo that you can share, which reproduces the problem you’re experiencing.
Did you install Hugo using the version in the Debian repos (and are not on unstable)? If so, you probably have too old a version and need to install using the .deb from Hugo releases, or the Linux binary tarball and put hugo somewhere in your PATH. On a default install of Debian with a GNOME you can put it in .local/bin/ and it will be in the PATH when you launch a new terminal.
The unformatted text also surprised me. For those that land here too…
I believe the explanation is: When browsing just the main page the post text is a summary without formatting (due to the theme maybe?). Clicking the “read more” button gives the actual post page where the text is formatted as expected.
To clarify other replies: At this point in the Quick Start there is only a local repository (the site exists only on the local machine), nor public directory (the site has not been published and the “Environment: development” server is “Serving pages from memory” on the local host).