It looks like the IAM policy isn’t actually attached to the role when you run the validation. From your command output, the role name is nautilus-app-role. Make sure that the policy nautilus-app-policy is attached to this role before applying Terraform. You can do that by running:
aws iam attach-role-policy --role-name nautilus-app-role --policy-arn arn:aws:iam::000000000000:policy/nautilus-app-policy
After attaching, verify again with:
aws iam list-attached-role-policies --role-name nautilus-app-role
Once you confirm the policy is listed, re-run your Terraform or validation step, the error should be resolved.