I think `opam` failed to find `jbuilder` on the `$PATH`, even though (presumably) `opam` has built it and installed it in it's own `bin/` directory. Try running

I think `opam` failed to find `jbuilder` on the `$PATH`, even though (presumably) `opam` has built it and installed it in it's own `bin/` directory. Try running `eval $(opam config env)` to update the `$PATH` and trying the commands again.