I installed LR6 on a new Dell notebook running latest version Windows 10. Import images and Library functions work well but when I open the Develop tab the frame where the image normally appears comes up as a uniform aqua green/blue filled frame with no image at all. I have tried uninstalling and reinstalling but no luck. When I installed an earlier version of LR5.7 it works fine so I suspect there is an incompatibility between my version of LR6 (although I have downloaded the current version) and the version of Windows 10 on my laptop.
This seems to be due to graphics processor.
Go to Lightroom Preferences > Performance > uncheck "use graphics processor"