Tethering Nikon & Leica cameras using Lightroom CC 2015/Lightroom 6 is not currently supported on El Capitan.
We recommend customers who rely on Lightroom for tethering Nikon and Leica cameras not upgrade to El Capitan at this time.
Our engineering team is working with our partners to correct this issue and provide compatibility in the future.
Do you have any timeframe when this issue will be resolved ?
See the official response from Adobe here: Lightroom: Can't use tethering with Nikon and Leica after installing MacOS 10.11 El Capitan. In the past, new releases of LR have come out about once every three months, plus or minus. LR CC 2015.3 / 6.3 came out Nov 17, 2015.