Since I already have the CS5.5, I bought the download version of lightroom 6 for my MacBook Pro running El Capitan 10.11.6 instead of a CC subscription.
After spending a day trying to get the actual installer to run (and yes I went through the forums, un-installed an old trial version of CC), I finally got the installer to start up. The new issue is that the windows are mostly blank. By that I meant there are some dashes where there should be text and buttons with no text. WTF? I can't seem to find anyone who has had a similar issue.
You didn't need to uninstall the CC version. LR 6 and CC is the Exact same program, same download and same file get installed on your hard drive. The difference comes from how you sign in.
As to your current problem is sounds like you have some type of computer problem.
If there's a computer problem, it's something that is escaping me. I've been using Macs since 1986 and I'm pretty good at trouble shooting. I didn't have lightroom CC. I had an older trial version of CC with photoshop. I bought the creative suite (minus lightroom) at an education discount but stayed with Aperture until it was no longer supported. I have been using the native Mac photo program, but I really needed something better so I bought the stand alone Lightroom 6. After cleaning up the older CS programs, un-installing the old trial CC (without Lightroom), running some diagnostics, etc. I find it hard to believe that I'm the only one that has ever had this problem of the blank installation window. Thanks for your comment.