Monitor install failure

I am trying to set up monitor node on a Ubuntu 18.04.6, but running into this error:

21:57:53 24571 ERROR Installing pg_auto_failover monitor in existing PostgreSQL instance at "/var/lib/postgresql/13/postgres3_sandbox" running on port 5432 is not supported.

verbose output:

$ pg_autoctl create monitor --hostname sb-sand-postgres103 --pgport 5432 --pgctl /usr/lib/postgresql/13/bin/pg_ctl --auth tr
ust --no-ssl -vv
22:10:45 25366 WARN  pgsetup.c:1747: No encryption is used for network traffic! This allows an attacker on the network to read all replication data.
22:10:45 25366 WARN  pgsetup.c:1749: Using --ssl-self-signed instead of --no-ssl is recommend to achieve more security with the same ease of deployment.
22:10:45 25366 WARN  pgsetup.c:1751: See https://www.postgresql.org/docs/current/libpq-ssl.html for details on how to improve
22:10:45 25366 INFO  pgsetup.c:1759: Using default --ssl-mode "prefer"
22:10:45 25366 DEBUG monitor_config.c:256: Reading configuration from /var/lib/postgresql/.config/pg_autoctl/var/lib/postgresql/13/postgres3_sandbox/pg_autoctl.cfg
22:10:45 25366 DEBUG pgsetup.c:122: pg_setup_init: /usr/lib/postgresql/13/bin/pg_ctl version 13.4
22:10:45 25366 DEBUG pgctl.c:212: /usr/lib/postgresql/13/bin/pg_controldata /var/lib/postgresql/13/postgres3_sandbox                                                  22:10:45 25366 ERROR monitor_pg_init.c:98: Installing pg_auto_failover monitor in existing PostgreSQL instance at "/var/lib/postgresql/13/postgres3_sandbox" running o
n port 5432 is not supported.

from running this command:

$ export PGDATA=export PGDATA=/var/lib/postgresql/13/postgres3_sandbox/monitor
$ pg_autoctl create monitor --hostname sb-sand-postgres103 --pgport 5432 --pgctl /usr/lib/postgresql/13/bin/pg_ctl --auth trust --no-ssl

postgres version:

postgres=# SELECT version();
                                                             version                                                             
---------------------------------------------------------------------------------------------------------------------------------
 PostgreSQL 13.4 (Ubuntu 13.4-4.pgdg18.04+1) on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0, 64-bit
(1 row)

pg_autoctl version:

$ /usr/bin/pg_autoctl --version
pg_autoctl version 1.6.2
pg_autoctl extension version 1.6
compiled with PostgreSQL 13.4 (Ubuntu 13.4-1.pgdg18.04+1) on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0, 64-bit
compatible with Postgres 10, 11, 12, 13, and 14

I saw another issue where they had a similar error, but I am not sure if its the same root cause as postgres installed on the node is from PGDG pacakage.

In the repo readme it states that you can install pg_auto_failover on an existing postgres setup on a primary node. Is this the case for a monitor node as well?

hapostgres / pg_auto_failover

Monitor install failure #823