Hey @Meer,
Add the following statement to the permissions policy for the IAM entity that needs to create the service-linked role:
{
    "Effect": "Allow",
    "Action": [
        "iam:CreateServiceLinkedRole",
        "iam:PutRolePolicy"
    ],
    "Resource": "arn:aws:iam::*:role/aws-service-role/ecs.amazonaws.com/AWSServiceRoleForECS*",
    "Condition": {"StringLike": {"iam:AWSServiceName": "ecs.amazonaws.com"}}
}