You can find the system requirements for LR6/LRCC listed on this page:
For WIndows: Windows 7 with Service Pack 1, Windows 8, Windows 8.1, or Windows 10
For Mac: Mac OS X 10.8, 10.9, 10.10, or 10.11. Lightroom CC 2015.2/Lightroom 6.2 is supported only on Mac OS X 10.9 and later.
So, if you're running on WIndows Vista or XP, or Mac OS X 10.8 or earlier, time to upgrade!
Hope that helps...
Select the version you have and make sure your computer meets minimum requirements to run the Product