[Solved]Unbound module Rpc_parallel

I tried to compile the simple.ml example of rpc_parallel by:

$ jbuilder build simple.exe
File "simple.ml", line 13, characters 54-77:
Error: Unbound module Rpc_parallel

I’d installed Rpc_parallel before

$ opam show --field=version rpc_parallel

Is there anything else I missed?

What does your jbuild file look like?

I got the same issue on my macOS laptop