It takes a while for the Artifact Repository to go from red to green in the Talend Big Data Platform Administration Center configuration tab.
Just as a heads up to others. After the cloudformation script runs and you can access the TAC page. I thought that it wasn't connecting to the artifact repository and I kept rolling back the deployment. This last time, I got called in to a meeting and it ran for a while. When I came back it had gone from red to green. I think it might take a while for the Nexus server to come online, so give it a while.