By default users other than root cannot open ports below 1000, for security reasons. If you want to - say - run your website in Elixir without nginx on port 80 or 443, you can allow the Erlang binary to open ports below 1000 using setcap (run as root):

setcap 'cap_net_bind_service=+ep' <path_to_erlang_folders>/erts-11.1/bin/beam.smp