You can’t install Lightroom CC 2015 and LR6 on the same machine. It’s one or the other.
I would suggest using the Uninstaller to trash whatever you have installed.
Click on the Apple icon top left and shut down and re-boot.
Launch Creative Cloud and wait for it to update if necessary.
Then go to the Apps tab and click the Install button (if you have a subscription) or the Try button if you wish to buy.
Perhaps I wasn't clear. I wasn't trying to install both. Either one would do, but neither one would install, both generating the same error. There is no uninstaller for Lightroom 6 or CC, because neither one installed. I tried the CC version first, and when that failed to install because of the above error DF024, I downloaded the 6 version and tried to install that. Same result -- nada -- same error code. I've done all the usual attempts at fixing the problem. Signed out an in of CC, rebuilt disk directory with Disk Warrior, run Apple repair disk, cleaned out all the caches, etc. Still won't install. Did a Safe Boot -- still won't install.
The next step is to reinstall the Yosemite OS (its a Mac), but I'm trying to find out if there is an easier solution, because if that doesn't work, I'll have to go through the PITA of reformatting the whole disk and restoring it, which I'm trying to avoid.
Unfortunately, no. I'd looked at this entry before. The DF024 error in the linked knowledgeable refers to color profiles in the user library, and the solutions involve making sure a aliased folder is int he location, not a actual folder. Not the problem with the error I got. The DF024 error I got, as noted above, refers to a "library_window.nib" in the lightroom application package itself. I've installed LR CC on another computer, my laptop, without a problem, so clearly the problem is in my iMac. But how to fix it in the least intrusive way is the question. Apparently this particular errors hasn't popped up on Adobe's radar often enough to merit inclusion in the knowledgeable.