MacOS hello world won't working

[ghost]

nomore@appledeMac-mini ~/workspace/private/ocaml/hello_world $ dune build hello_world.exe
File "dune", line 3, characters 12-20:
3 | 	(libraries lwt.unix))
                ^^^^^^^^
Error: Library "lwt.unix" not found.
Hint: try:
  dune external-lib-deps --missing hello_world.exe
nomore@appledeMac-mini ~/workspace/private/ocaml/hello_world $ dune external-lib-deps --missing hello_world.exe
Error: The following libraries are missing in the default context:
- lwt.unix
Hint: try:
  opam install lwt
nomore@appledeMac-mini ~/workspace/private/ocaml/hello_world $ opam install lwt
[NOTE] Package lwt is already installed (current version is 5.5.0).
nomore@appledeMac-mini ~/workspace/private/ocaml/hello_world $ dune build hello_world.exe
File "dune", line 3, characters 12-20:
3 | 	(libraries lwt.unix))
                ^^^^^^^^
Error: Library "lwt.unix" not found.
Hint: try:
  dune external-lib-deps --missing hello_world.exe
nomore@appledeMac-mini ~/workspace/private/ocaml/hello_world $ cat dune
(executable
	(name hello_world)
	(libraries lwt.unix))
nomore@appledeMac-mini ~/workspace/private/ocaml/hello_world $ cat hello_world.ml
Lwt_main.run (Lwt_io.printf "Hello, world!\n")

[rgrinberg]

Did you select the opam switch and then eval the environment?

Something like $ opam switch 4.13.1 followed by $ eval $(opam env).