Comment 3 for bug 1913594

This file is provided by:
postgresql-client-common: /usr/share/postgresql-common/maintscripts-functions

And the one you configure has a dependency:
apt-cache show postgresql-12 | grep commo
Depends: locales | locales-all, postgresql-client-12, postgresql-common (>= 182~)

Therefore postgresql-client-common should be unpacked before postgresql-12 is configured.
I wonder if there are any 3rd party packages and/or repositories active.

Could you report:
$ apt-cache policy postgresql-client-common postgresql-12

Also the bug report misses the usually attached console output of the install/upgrade that would have shown the ordering of the two packages - if you can reproduce this and attach the console output please?