Flymake is only useful if at least one backend is configured to
provide the buffer-checking service. This is done via the hook
flymake-diagnostic-functions. See Hooks in The
Emacs Editor.
It’s possible that some major modes or a third-party package has
already setup this hook for you, by adding backend functions to
flymake-diagnostic-functions. If you know Elisp you may also
write your own Flymake backend functions. See Backend functions.