I’m not using use-package, therefore I can only guess.

I can’t see here that you associate flycheck-mode to elixir-mode. So unless you do global-flycheck-mode anywere, you need to (add-hook 'elixir-mode-hook flycheck-mode) at a place where it is appropriate, maybe use-package has some special syntax for it.

PS: Even though I generally prefer emacs over everything else, I currently use VScode for elixir, because language server support is so much better. I hope LS support will be available in alchemist.el soon!

@dimitarvp yes I do and below is my config:

;; Elixir
(after! lsp-mode
  (defun dap-elixir--populate-start-file-args (conf)
    "Populate CONF with the required arguments."
    (-> conf
        (dap--put-if-absent :dap-server-path `("~/lang_servers/elixir-ls/release/debugger.sh"))
        (dap--put-if-absent :type "mix_task")
        (dap--put-if-absent :name "mix test")
        (dap--put-if-absent :request "launch")
        (dap--put-if-absent :task "test")
        (dap--put-if-absent :taskArgs (list "--trace"))
        (dap--put-if-absent :projectDir (lsp-find-session-folder (lsp-session) (buffer-file-name)))
        (dap--put-if-absent :cwd (lsp-find-session-folder (lsp-session) (buffer-file-name)))
        (dap--put-if-absent :requireFiles (list
                                            "lib/**"
                                            "test/**/test_helper.exs"
                                            "test/**/*_test.exs"))))

  (dap-register-debug-provider "Elixir" 'dap-elixir--populate-start-file-args)
  (dap-register-debug-template "Elixir Run Configuration"
                              (list :type "Elixir"
                                    :cwd nil
                                    :request "launch"
                                    :program nil
                                    :name "Elixir::Run"))

  ;; Enable formatting on save and send reload command to Elixir’s REPL
  (add-hook 'elixir-mode-hook
            (lambda ()
              (add-hook 'before-save-hook 'lsp-format-buffer nil t)
              (add-hook 'after-save-hook 'alchemist-iex-reload-module))))


;; Configure ExUnit package
(use-package! exunit)

;; Disable popup quitting for Elixir’s REPL
(set-popup-rule! "^\\*Alchemist-IEx" :quit nil :size 0.3)

Cheers