setting up postgres replication FATAL: database system identifier differs between the primary and standby

Ahoy there,

I am attempting to set up postgres12 with replication between two servers.

I followed these instructions.

https://www.digitalocean.com/community/tutorials/how-to-set-up-physical-streaming-replication-with-postgresql-12-on-ubuntu-20-04

At step 4 - I restarted the replica database with command "sudo
systemctl restart postgresql(at)12-main"

I see these messages in the /var/log/postgresql/postgresql-12-main.log file

--- snip ---

2022-02-12 17:39:51.345 UTC [237364] LOG: entering standby mode
2022-02-12 17:39:51.356 UTC [237365] FATAL: database system identifier
differs between the primary and standby
2022-02-12 17:39:51.356 UTC [237365] DETAIL: The primary's identifier
is 7048326921585830819, the standby's identifier is 7063458841707763655.
2022-02-12 17:39:51.367 UTC [237369] FATAL: database system identifier
differs between the primary and standby
2022-02-12 17:39:51.367 UTC [237369] DETAIL: The primary's identifier
is 7048326921585830819, the standby's identifier is 7063458841707763655.
2022-02-12 17:39:56.380 UTC [237371] FATAL: database system identifier
differs between the primary and standby
2022-02-12 17:39:56.380 UTC [237371] DETAIL: The primary's identifier
is 7048326921585830819, the standby's identifier is 7063458841707763655.

--- snip ---

Is there an easy way to solve this problem?

Cheers