The target machine has none of the earlyer used keypairs when replicating TO the machine.
As mentioned multiple times now, your remote system needs the public key corresponding to the key pair configured.
Think of it this way: adding your public key to the remote machine installs a door lock which your private key on the source machine can open. If the wrong or no public keys are configured on the destination, your source system cannot open/access anything there.
edit: Sorry if I misunderstood you, yes the generated keypair is only visibile on the source machine in this case, I thought you meant you do see the pub key of the key pair on your remote system.
Since the semiautomatic approach failed, refer to the manual approach in the documentation. I looked over it and it seems rather straightforward, insert the public key at the corresponding places.
I'm failing to understand what the problem is actually, since you already had ssh connections configured before.
edit:
So for the sake of completeness:
I assume this is the keypair you want to use and that is configured with your replication tasks:
On the source machine go to System > SSH Keypairs
Select and then edit (?) your Runonce3 pair. Download the pub key.
Now, this is what you didn't seem to have done so far:
Log in to the TrueNAS system that needs to register the public key. Go to
Accounts > Users and edit the
root account. Paste the SSH public key text into the
SSH Public Key field.
Add your key here: