Nvim healthcheck doesn't work

I installed this plugin in Neovim with Lazy, it's installed fully according to Lazy, however

:SMLHealthCheck or nvim +SMLCheckHealth

bettersml: health#bettersml#check

- ERROR The healthcheck report for "bettersml" plugin is empty.

I also installed it in Vim, there I get:


## REPL
  - error: No REPL backend available.
      - To use the SML/NJ REPL, you must either be using Neovim
      - or be using tmux and have benmills/vimux installed.
  - ok: SML/NJ is executable at 'sml'.
  - ok: rlwrap is available and will be used to launch REPLs
## Type information
  - ok: MLton is executable at 'mlton'.
  - ok: Support files are executable at '/home/cat/.vim/plugged/vim-better-sml/b
in/vbs-util'.
## General

etc

The REPL does work in Nvim and the keymaps also work.

NVIM v0.11.0-dev-85+g93c55c238
Build type: RelWithDebInfo
LuaJIT 2.1.1713484068

jez / vim-better-sml

Nvim healthcheck doesn't work #23