Thanks @rubi1200
I was trying to follow the instructions from https://documentation.ubuntu.com/project/maintainers/AA/aa-i386/#how-to-expand-i386-port-scope
Even though the discussion in Community process for 32-bit compatibility is old, it still seems relevant?
(But please tell if I should use some other thread instead)
Note that even though that discussion may appear old, as some packages were added then automatically as they were required dependencies, they may not have been requested manually as nobody realized they were missing.
As those dependencies change over time, we may find ourselves needing to request packages to be added manually, as was the case here.