Comment 11 for bug 1533587

So we're dealing with some system which appears to be detected as something different than we're used to then. From what I can tell the partman-prep package would get installed if the subarchitecture is correctly detected as chrp_ibm or prep, etc.

AIUI this would come from archdetect.

Could you please run the archdetect command in a terminal on the system, and tell us here what it reports? The result should be in the form 'ppc64el/subarchitecture', I somewhat expect that the subarchitecture on this system is reported as 'unknown'.