Ops Manager is talking to AWS, so if it's not able to login then there's something wrong with the IAM set up. Double check your IAM set up with the instructions here -> https://docs.pivotal.io/pivotalcf/2-3/om/aws/prepare-env-manual.html#create-iam.
For best results, Pivotal recommends installing using Terraform. This automates the set up of AWS infrastructure including all the IAM rules and uses known good and tested configuration. This provides the smoothest installation experience.
https://docs.pivotal.io/pivotalcf/2-3/customizing/aws-terraform.html