Adobe uses Nikon software to do tethering with LR. As of El Capitan, Apple removed older USB functions that Nikon was using for their tethering. Apparently this removal took Nikon by surprise, because the official Nikon software quit tethering as well, even though the OS beta had been available for months as was information about the USB functions being removed. Nikon took 6 months to fix things, long after LR 5.7.1 had become obsolete, so LR 6.4/LR CC 2015.4 is the oldest LR version that will tether Nikon cameras on El Capitan and newer Mac OSes:
Adobe has discovered the compatibility issues listed below with running Lightroom CC 2015 or Lightroom 6 with Mac OS 10.…
Only an OS downgrade or a LR pay-for upgrade will allow you to tether your Nikon on your Mac, again, with LR.