Your database files were created by PostgreSQL version 13 OS is openSUSE Tumbleweed. Previously I had PostgreSQL 13 installed. I just removed it by:
&gt; sudo zypper rm 'postgres*'

&gt; sudo zypper rr PostgreSQL

&gt; sudo rm -rf /usr/local/var/postgres

Then I install PostgreSQL 12 by instructions here:
&gt; sudo zypper addrepo http://download.opensuse.org/repositories/server:database:postgresql/openSUSE_Tumbleweed/ PostgreSQL

&gt; sudo zypper ref

&gt; sudo zypper in -f postgresql postgresql-server postgresql-contrib

&gt; sudo zypper in -f postgresql-plperl postgresql-plpython postgresql-pltlc

&gt; sudo systemctl enable postgresql

&gt; sudo systemctl start postgresql

But the database status is faulty:
&gt; sudo systemctl status postgresql.service  
[sudo] password for root:  
● postgresql.service - PostgreSQL database server
     Loaded: loaded (/usr/lib/systemd/system/postgresql.service; enabled; vendor preset: disabled)
     Active: failed (Result: exit-code) since Thu 2021-05-27 23:08:43 EDT; 2h 10min ago
    Process: 1576 ExecStart=/usr/share/postgresql/postgresql-script start (code=exited, status=1/FAILURE)
        CPU: 21ms

May 27 23:08:43 localhost systemd[1]: Starting PostgreSQL database server...
May 27 23:08:43 localhost postgresql-script[1576]:  Your database files were created by PostgreSQL version 13.
May 27 23:08:43 localhost postgresql-script[1576]:  Could not find executables for this version.
May 27 23:08:43 localhost postgresql-script[1576]:  Please install the PostgreSQL server package for version 13.
May 27 23:08:43 localhost systemd[1]: postgresql.service: Control process exited, code=exited, status=1/FAILURE
May 27 23:08:43 localhost systemd[1]: postgresql.service: Failed with result 'exit-code'.
May 27 23:08:43 localhost systemd[1]: Failed to start PostgreSQL database server.


For some reason, whatever I try, I cannot get rid of these status errors:

Your database files were created by PostgreSQL version 13.
Could not find executables for this version.
Please install the PostgreSQL server package for version 13.

How can I install PostgreSQL 12 and get rid of PostgreSQL 13 database files?
I decided to abandon the idea of downgrading from 13 to 12. I reinstalled Pg 13 and am sticking with it.
I work with a platform that recommends Pg 12. However, I won't come back to 12 unless something goes wrong with 13.