The installer is running from a 32-bit environment, the installer still offers 64-bit ISO's and other install media (where available) as possible install sources. This is wrong because the installer will happily install all the packages, but it cannot finish the install process because it needs to run some code from the installed system. Being a 32-bit kernel, it cannot chroot into the installed system.
NOTE: This is not an issue the other way around. The 64-bit ISO should be allowed to install 32 and 64-bit OS's