On Debian and derivatives like Ubuntu, the bash manual is not installed by default, or only in the man format (and info falls back to displaying the man page when the info manual is not available) which is hardly usable for a manual of this size.
You need to install the bash-doc package first.
apt install bash-doc
(as root).
Same goes for most manuals in info format. For instance, you'd need to install zsh-doc, gdb-doc, glibc-doc, gcc-doc, gawk-doc to get manuals of those large software in more useful formats.
More generally, the software packages come with minimal documentation in man format, a few documents in /usr/share/doc/<package-name>, but larger documentation in other format (can be info, html...), when available are supplied in a <package-name>-doc package. That makes sense as users don't necessarily need the documentation, especially when software are installed as a dependency of another package and the users will never get to use them directly.
For some software libraries, the API documentation (generally in man format) is often supplied in the <libpackage-name>-dev package (again users are unlikely to need that information unless they're going to develop software for it), though again a -doc package can complete it with richer format/content.
