Same thing under Windows. Seems like this is an Adobe wide problem now.
Try quitting and restarting the creative cloud app. If that doesn't do anything, try rebooting your machine. Lots of people are having this issue. It is not clear to me what is causing it. It worked right away on my Mac with Yosemite.
No joy with either quitting Creative Cloud or rebooting the computer - I've done all the normal stuff and none of it works. In particular, I was hoping a reboot would clear something in the way but no ...
I've got most applications turned off and just running the nominal set of processes that come up with Yosemite. Again, no joy - the new LR has some interesting incompatibility that is halting startup early on.
Sorry for the trouble with first launch.
If on Mac, try signing out of the Creative Cloud Desktop application, then sign back in: helpx.adobe.com/creative-cloud/kb/sign-in-out-creative-cloud-desktop-app.html
If on Win and signing out and back in does not resolve, try renaming both the SLCache and SLStore folders prior to launching Lightroom?
Locations of both of these folders are located in Solution 3 of this document: https://helpx.adobe.com/lightroom/kb/unable-launch-lightroom-55.html
Let us know how it goes
Can you take a look at console.app (just type console in the search looking glass on the right top. See if there is a Adobe Photoshop Lightroom CC report in the User Diagnostic Reports section of in the System Diagnostics section. There might be a clue in those. Also try deleting the user preferences for Lightroom 6. You'll find those in ~/Library/Preferences/ This folder is normally hidden but can be revealed by hitting command-shift-G in Finder and pasting ~/Library/Preferences/ in the dialog you'll get. The file is called com.adobe.Lightroom6.plist and com.adobe.Lightroom6.LSSharedFileList.plist Just drag those in the trash and see if Lightroom will start.
Thanks, that solved the problem. Lesson learned, for me, was that quitting creative suite is not logging out - that did the trick.