I have the same issue as jim (above) and tried logging Out and back In through Creative Cloud as you mentioned but still have the same issue. Tried restarting computer as well.
I'm running a Mac with OS10.10.3
It only affects the new install of Lightroom CC 2015. I can still open Lightroom 5. Both apps I installed through the CC app
I have the same issue. I have uninstalled and re installed at least 5 times so far. The lightroom that i used to use is also uninstalled, which was working by the way and now i am left with nothing, and i have tried logging off from creative cloud and tried launching from my launchpad but still nothing.
I am using an Mac with OS X El Capitan 10.11.6