Why can't I enable the Design Space technology preview?

Design Space technology preview in Photoshop CC 2015 version 16.0 (released June 2015) is enabled in Preferences > Technology Previews. To enable Design Space:

1. You must be running 64-bit Windows 8.1 or Mac OS X 10.10;
2. You must have graphics acceleration enabled in Preferences > Performance; and
3. You must not be running the French (France) language version of Photoshop 16.0. (A workaround for this problem is described here.)

Note that the Design Space technology preview is only available in English.

Will Design Space be enabled on older operating systems like Windows 7?

Older operating systems, like Windows 7 and earlier and Mac OS X 10.9 and earlier, have many severe bugs that prevent Design Space from working correctly. We understand that many customers use older operating systems, and we're still investigating whether it could be possible to run Design Space on these earlier operating systems. We're generally more focused on building technology that makes full use of the features of modern operating systems, however.