The Debian package 'preload' is (now) missing from Debian "unstable/sid". [Most likely removed?!]
This means that the ISO-build fails until either the package returns, is added to 'extra' or 'fixes', or is removed from the list of packages to install. [I have looked it up: there is no package depending on 'preload' at this time.]
Yes, the 'preload' package has been removed (and is unlikely to return). There's only an unofficial port for 'sid' available. The
package hadn't been maintained by its developer for years and, recently, it caused more problems than it solved. It originates
from a time when the daemon was used to proactively load packages from slower storage devices (like a hard drive). Today, 'preload'
offers no benefit and is therefore no longer needed. It should simply be removed from the installation list.
edlin