Command /Applications/Xcode-beta.app/Contents/Developer/usr/bin/actool failed with exit code 255

That happened because of JPG images in assets catalog, so I imported jpg images directly in the project, and now it works

I had the same issue with Xcode V7b6 and JPG images in assets. Strangely this was working for some JPGs but not for others.

Then I've looked at the JPGs and their color profile, exif data and other meta stuff and indeed removing these data (via GraphicConverter) solved it for us and Xcode now builds fine.