If you’ve updated to macOS Mojave, you’ll likely see a warning the first time you generate for the Kindle platform:
Vellum makes use of Amazon’s KindleGen tool to convert ebooks to their proprietary
mobi format. When running Mojave, your Mac will warn about this program because it has not been built in a 64-bit format, something Apple will require in future versions of macOS.
Until they do, you will periodically see this warning when you generate for Kindle. This does not affect your generated books. We recognize it can cause concern, though, so we are investigating ways to create ebooks for Kindle without triggering this warning.
Update 10/2/19: Vellum 2.6.2 uses Kindle Previewer, which has been updated for current versions of macOS.