Problem with Migration Assistant

I'm doing the multi-step job of upgrading from an old system (old Mac Pro on 10.6.8) to a new one (late-teens model of Mac Pro. Without losing my old files. I know this needs to be done in steps: to Mountain Lion to El Capitan to Catalina (my endpoint for the moment).

Currently I have my stuff on the old Mac Pro, which I have upgraded to Mountain Lion. I am trying to transfer it to another machine on which I have El Capitan running, with no users defined except for one temporary Admin account.

I have run a TM dump of this system onto an external tape, because I had oddly nasty results trying to migrate via a direct computer-to-computer coupling. The TM worked, and incidentally I ran both computers through Apple Store's update for their respective OS levels.

The point, at last: "This user needs attention before Migrating" appears in red type when I tell it to migrate from the TM tape. Huh? What means this? Is there a source I can go to? Google, of course, has little to say except pointing to a not quite relevant query on this forum.