Update: Shortly after posting, the Windows Insider team updated their blog post to say that the 32-bit machine bug is fixed, and that 32-bit machine users can now download and install Windows 10 Insider build 15042 just fine.
Although Microsoft is releasing Windows 10 Insider build 15042 for PC (and 15043 for mobile), there is a bug in this latest build that “impacts our ability” to release the build for 32 bit machines. Instead of waiting on a fix that would only affect a few Insiders, Microsoft decided to release the build anyway:
We have discovered a bug that impacts our ability to release this build to PCs running 32-bit (x86) versions of Windows 10 Insider Preview builds. Because the majority of Insiders in the Fast ring have PCs running 64-bit (x64) – we decided to go ahead and release this build with 32-bit PCs blocked until our next flight.
So if you’re on a 32 bit PC, you won’t see this latest Insider build, even if you’re on the Fast Ring. You’ll have to wait this round out.
For everyone else, however, build 15042 for PC and 15043 for mobile are available for download.