It looks like MacOS needs the WiFi to find your location. I had disabled by WiFi adapter/service because I have my iMac hard wired with ethernet.
Turn on your WiFi: System Preferences > Network > highlight 'WiFi' in left pane > click gear icon at the bottom of left pane > choose 'Make Service Active' > right pane, choose 'Turn WiFi On' > connect to a WiFi network.Make sure system services have location permissions enabled: System Preferences > Security & Privacy > bottom left corner, click the lock to unlock, enter password > Privacy tab > right pane, scroll to the bottom to System Services > click 'details' button > check everything > done.I had to do 2 things to get Night Shift working (macOS 10.13.6):
