Upgrade Process Says "The database server was not shut down cleanly" but it was

I'm in the process to update my Postgres 10 to 12 (it's my first server
upgrade). I've shutdown my Postgres 10 using "systemctl stop
postgresql-10.service" and "systemctl status postgresql-10.service" says:
Active: inactive (dead) since Sun 2020-05-03 22:51:46 CEST;

After that I've started the upgrade process with:
/usr/pgsql-12/bin/pg_upgrade --old-bindir=/usr/pgsql-10/bin/
--new-bindir=/usr/pgsql-12/bin/ --old-datadir=/var/lib/pgsql/10/data
--new-datadir=/var/lib/pgsql/12/data --old-options '-c
config_file=/var/lib/pgsql/10/data/postgresql.conf' --new-options '-c
config_file=/var/lib/pgsql/12/data/postgresql.conf'

And the output is:
Performing Consistency Checks
-----------------------------
Checking cluster versions ok
Checking database user is the install user ok
Checking database connection settings ok
Checking for prepared transactions ok
Checking for reg* data types in user tables ok
Checking for contrib/isn with bigint-passing mismatch ok
Checking for tables WITH OIDS ok
Checking for invalid "sql_identifier" user columns ok
Creating dump of global objects ok
Creating dump of database schemas

ok
Checking for presence of required libraries ok
Checking database user is the install user ok
Checking for prepared transactions ok

If pg_upgrade fails after this point, you must re-initdb the
new cluster before continuing.

Performing Upgrade
------------------
Analyzing all rows in the new cluster ok
Freezing all rows in the new cluster ok
Deleting files from new pg_xact ok
Copying old pg_xact to new server ok
Setting next transaction ID and epoch for new cluster ok
Deleting files from new pg_multixact/offsets ok
Copying old pg_multixact/offsets to new server ok
Deleting files from new pg_multixact/members ok
Copying old pg_multixact/members to new server ok
Setting next multixact ID and offset for new cluster ok
Resetting WAL archives ok
Setting frozenxid and minmxid counters in new cluster ok
Restoring global objects in the new cluster ok
Restoring database schemas in the new cluster

ok
Copying user relation files

ok
Setting next OID for new cluster
*failure*

Consult the last few lines of "pg_upgrade_utility.log" for
the probable cause of the failure.
Failure, exiting

The pg_upgrade_utility.log shows:
command: "/usr/pgsql-12/bin/pg_resetwal" -o 1091293 "/var/lib/pgsql/12/data"
>> "pg_upgrade_utility.log" 2>&1
The database server was not shut down cleanly.
Resetting the write-ahead log might cause data to be lost.
If you want to proceed anyway, use -f to force reset

1. I did shutdown the server properly "systemctl stop
postgresql-10.service", is there a better way to do that?
2. Where do I set the "-f" flag if I choose to force reset?

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