Hi,
you need to accept the remote host key of the monitored System on the checkmk server, If it is only one system you can create a ssh session from inside your checkmk site (as the Site User) with the same command you are using, but with the variables changed for the real values.
An insecure option could be to add
-o StrictHostKeyChecking=no to the ssh command.
Ok, we wrote at the same time 