“softwareupdate -l” on Mac terminal doesn't return anything
Do you have a custom software update server set? See Reset OS X Software Update server for info on how to check &amp; if necessary reset it.

Another option is to bypass the update server system by downloading and installing the update manually. You can find updates at support.apple.com/downloads; the specific one you need is the macOS Sierra 10.12.6 Combo Update

(Note: the 12.12.6 Combo update should not be confused with the "macOS Sierra 10.12.6 Update" -- the non-combo or "delta" version requires that you already be up to 10.12.5.  You're running 10.12.4, so you need the combo update.)