EKS not able to authenticate to Kubernetes with Kubectl - "User: is not authorized to perform: sts:AssumeRole"

Your policy is wrong. User can’t assume another IAM user. It should be something like this:

{
"Version": "2012-10-17",
"Statement": [
    {
        "Effect": "Allow",
        "Action": [
            "sts:AssumeRole"
        ],
        "Resource": "arn:aws:iam::1234:role/prod-Eks-1234-admins"
    }
]
}