This module is deprecated and superseded by mod_smacks. If you explicitly disabled mod_offline you need the new module mod_nooffline_noerror to regain all features of this deprecated module.
With the plugin installer in Prosody 0.12 you can use:
sudo prosodyctl install --server=https://modules.prosody.im/rocks/ mod_smacks_noerror
For earlier versions see the documentation for installing 3rd party modules