scp between 2 AWS Instances

I have 2 AWS instances of Ubuntu 14.04. One instance is the master ELK server. The other instance will be the log forwarder for logstash. As part of the online instructions to setup the forwarder I enter the following on the ELK server instance:

scp /etc/pki/tls/certs/logstash-forwarder.crt user@http://client_server_private_address:/tmp

I replaces user with ubuntu and the private address with the public DNS name for the Log Forwarder instance.

Both are in the same VPC and Security Group. When I run scp -v i get: No more authentication methods to try. Permission denied (publickey).

Any ideas would be greatly appreciated! I will continue to google...

Lew