Unable to assume role and validate the specified targetGroupArn

For me, the problem was that I forgot to attach the right policy to the service role. Attaching this AWS-managed policy helped: arn:aws:iam::aws:policy/service-role/AmazonEC2ContainerServiceRole

I was seeing an identical error message and I was doing something else wrong:

I had specified the loadbalancer's ARN and not the loadbalancer's target_group ARN.