STEPS TO REPRODUCE:

1. Install darcs with stack
2. Download darcs autocompletion file from
https://hub.darcs.net/darcs/darcs-2.16/browse/contrib/darcs_completion
to /etc/bash_completion.d/darcs
3. Do darcs <TAB> <TAB> in terminal emulator

Environment: Linux Mint 20

EXPECTED:

List of options (clone, --version, --help etc)

GIVEN:

Nothing

POSSIBLE CAUSE:

darcs do not implement --list-options

It works for me. When no command is given, the option darcs is invoked
with is called --commands, not --list-options. Outside of a darcs repo
this gives me

$ darcs --commands
help
initialize
clone
log
--help
--version
--exact-version

Have you checked that the completion script was downloaded correctly?
Downloading from the URL you posted gives me the html page that displays
the file, not the bash completion script itself. To get that, use

  wget http://darcs.net/screened/contrib/darcs_completion

works for me