When I hit C-x b to switch to buffer I receive:
Symbol's function definition is void: help--symbol-class.
I'm on Linux with Emacs 27.1 (build 1 - GTK 3.24.24)
1 Answer
The help--symbol-class function is a standard part of Emacs, but only since version 28.1, so you evidentially have code in your config which is incompatible with the version of Emacs you're using.
I suspect you're using some package which purports to be compatible with 27.1 but has inadvertently introduced some code which depends on newer versions. (And is explicitly calling a function which, by naming convention, isn't intended to be part of the API; but maybe there was no better alternative.)
M-x toggle-debug-on-error should track down the culprit.
(If you don't know how to proceed after triggering the debugger, then copy and paste the backtrace into your question.)
emacs -Q? If not, I suggest you figure out what command you have bound toC-x bby typingC-h k C-x b.