The error "Destination role is not a mapping target" is perhaps a little obscure, but it tells you what the problem is. LoadBalancerInstance.lb_service is a "source", i.e., a node in the ADDM data store. You need to specify a "target" -- a CI in the CMDB. If you look at the LoadBalancerInstance syncmapping, you will see that the CI created for lb_service is called service_ci.
So, change the first line of the body to
contained_ci := LoadBalancerInstance.service_ci;
and it should work fine.
Thank you Duncan.