Following the install instructions for the packages (https://www.openproject.org/docs/installation-and-operations/installation/packaged/), I've tried on 22.04, 20.04, and Debian 12.  It appears to install OpenProject 16.0.1, then installs Postgres v13 with the wrong encoding while making a reference to OpenProject v15.  

Setting up openproject (16.0.1-1749111325.7815b6b7.bookworm) ...
==============
The openproject package provides an installer. Please run the following command to finish the installation: openproject configure

I ask it to install Postgres and here is the log from the pre-install:

[legacy-installer] ./bin/preinstall
[openproject-edition] ./bin/preinstall
[postgres] ./bin/preinstall
INFO: autoinstall=install. A new PostgreSQL v13 server will be installed locally unless a conflicting installation is found.
INFO: Installing PostgreSQL v13...

The following NEW packages will be installed:
 libcommon-sense-perl libio-pty-perl libipc-run-perl libjson-perl libjson-xs-perl libllvm19 libsensors-config libsensors5 libtypes-serialiser-perl libz3-4 postgresql-13 postgresql-client-13 postgresql-client-common postgresql-common postgresql-common-dev sysstat

Creating config file /etc/postgresql-common/createcluster.conf with new version
Building PostgreSQL dictionaries from installed myspell/hunspell packages...
Removing obsolete dictionary files:
Created symlink /etc/systemd/system/multi-user.target.wants/postgresql.service -> /lib/systemd/system/postgresql.service.
Setting up postgresql-13 (13.21-1.pgdg120+1) ...
Creating new PostgreSQL cluster 13/main ...
/usr/lib/postgresql/13/bin/initdb -D /var/lib/postgresql/13/main --auth-local peer --auth-host md5
The files belonging to this database system will be owned by user "postgres".
This user must also own the server process.

The database cluster will be initialized with locale "C".
The default database encoding has accordingly been set to "SQL_ASCII".
The default text search configuration will be set to "english".

Success. You can now start the database server using:

   pg_ctlcluster 13 main start

Processing triggers for man-db (2.11.2-2) ...
Processing triggers for libc-bin (2.36-9+deb12u8) ...
waiting for server to shut down.... done
server stopped
pg_ctl: PID file "/var/lib/postgresql/13/main/postmaster.pid" does not exist
Is server running?
trying to start server anyway
waiting for server to start.... done
server started
ALTER ROLE
INFO: Checking connection to PostgreSQL server...  ?column? 
----------
       1
(1 row)

OK

[memcached] ./bin/postinstall
chown: warning: '.' should be ':': 'openproject.openproject'
[openproject] ./bin/postinstall
/opt/openproject/config/database.yml:30:in '<main>': already initialized constant DATABASE_OPTIONS (StructuredWarnings::BuiltInWarning)
/opt/openproject/config/database.yml:30:in '<main>': previous definition of DATABASE_OPTIONS was here (StructuredWarnings::BuiltInWarning)
/opt/openproject/config/database.yml:59:in '<main>': already initialized constant DATABASE_VARIABLES (StructuredWarnings::BuiltInWarning)
/opt/openproject/config/database.yml:59:in '<main>': previous definition of DATABASE_VARIABLES was here (StructuredWarnings::BuiltInWarning)
/opt/openproject/config/database.yml:63:in '<main>': already initialized constant DATABASE_DEFAULTS (StructuredWarnings::BuiltInWarning)
/opt/openproject/config/database.yml:63:in '<main>': previous definition of DATABASE_DEFAULTS was here (StructuredWarnings::BuiltInWarning)
INCOMPATIBLE DATABASE ENCODING DETECTED

Your database encoding is SQL_ASCII, which is incompatible with ICU
collation used in OpenProject v15.

Please check the instructions on how to change database encoding:
https://www.openproject.org/docs/installation-and-operations/misc/changing-database-encoding/

This check can be skipped by setting environment variable OPENPROJECT_SKIP_DB_ENCODING_CHECK=true

So I try the URL that shows how to change the encoding, except none of the commands work except for the backup command:

root@opdb:# openproject run backup
* Generating database backup... done
/var/db/openproject/backup/postgresql-dump-20250613041441.pgdump
* No SVN repositories folder. Ignoring.
* No Git repositories folder. Ignoring.
* Generating attachments backup... done
/var/db/openproject/backup/attachments-20250613041441.tar.gz
* Saving configuration... done
/var/db/openproject/backup/conf-20250613041441.tar.gz
root@opdb:# openproject config:get DATABASE_URL
postgres://openproject:l1OsZSc0S8u0yvmvpZ4VCmzfhDbk95IA@127.0.0.1:45432/openproject
root@opdb:# su postgres -c createdb -E UNICODE -O openproject openproject_backup
su: invalid option -- 'E'
Try 'su --help' for more information.
root@opdb:# psql 'postgres://openproject:l1OsZSc0S8u0yvmvpZ4VCmzfhDbk95IA@127.0.0.1:45432/openproject' -c 'CREATE DATABASE "openproject_backup" ENCODING UNICODE'
ERROR:  new encoding (UTF8) is incompatible with the encoding of the template database (SQL_ASCII)
HINT:  Use the same encoding as in the template database, or use template0 as template.