NLB is not able to return Security Group ID

I am trying to create NLB using cloudformation template and I want security group to be return. I am following this documentation but its giving error. Documentation: https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-elasticloadbalancingv2-loadbalancer.html

I have attached file for reference.

Error: Template format error: Every Value member must be a string.. Rollback requested by user.

{
  "AWSTemplateFormatVersion": "2010-09-09",
  "Description": "The template used to create an ALB Service.",
  "Parameters": {
    "SecurityGroupIDs": {
      "Type": "CommaDelimitedList",
      "Default": "sg-00cdd0ae543d7d743c"
    },
    "SubnetIDs": {
      "Type": "CommaDelimitedList",
      "Default": "subnet-07bcbafea8d1ae10fe,subnet-07839cd50992c3c05b"
    },
    "VpcID": {
      "Type": "String",
      "Default": "vpc-096ae9d52761a14542"
    },
    "LoadBalancerName": {
      "Type": "String",
      "Default": "ajt-nlb"
    },
    "ArnAlb": {
      "Type": "String",
      "Default": "arn:aws:elasticloadbalancing:us-west-2:account:loadbalancer/app/cv-ann-ALB/9fd999dfa42663a4a"
    },
    "TargetGroupName": {
      "Type": "String",
      "Default": "ajt-tg"
    }
  },
  "Resources": {
    "TargetGroup": {
      "Type": "AWS::ElasticLoadBalancingV2::TargetGroup",
      "Properties": {
        "HealthCheckPath": "/healthcheck",
        "Name": {
          "Ref": "TargetGroupName"
        },
        "Port": 80,
        "Protocol": "TCP",
        "HealthCheckProtocol": "HTTP",
        "VpcId": {
          "Ref": "VpcID"
        },
        "TargetType": "alb",
        "Targets": [
          {
            "Id": {
              "Ref": "ArnAlb"
            },
            "Port": "80"
          }
        ]
      },
      "DependsOn": [
        "LoadBalancer"
      ]
    },
    "LoadBalancer": {
      "Type": "AWS::ElasticLoadBalancingV2::LoadBalancer",
      "Properties": {
        "IpAddressType": "ipv4",
        "Type": "network",
        "Name": {
          "Ref": "LoadBalancerName"
        },
        "SecurityGroups": {
          "Ref": "SecurityGroupIDs"
        },
        "Subnets": {
          "Ref": "SubnetIDs"
        },
        "Scheme": "internal"
      }
    },
    "Listener": {
      "Type": "AWS::ElasticLoadBalancingV2::Listener",
      "Properties": {
        "DefaultActions": [
          {
            "Type": "forward",
            "TargetGroupArn": {
              "Ref": "TargetGroup"
            }
          }
        ],
        "LoadBalancerArn": {
          "Ref": "LoadBalancer"
        },
        "Port": 80,
        "Protocol": "TCP"
      }
    }
  },
  "Outputs": {
    "ArnNlb": {
      "Description": "The ARN of network load balancer.",
      "Value": {
        "Ref": "LoadBalancer"
      }
    },
    "Listener": {
      "Description": "The arn of listener.",
      "Value": {
        "Ref": "Listener"
      }
    },
    "TargetGroup": {
      "Description": "The arn of target group.",
      "Value": {
        "Ref": "TargetGroup"
      }
    },
    "Sg": {
      "Description": "The arn of listener.",
      "Value": {
        "Fn::GetAtt": ["LoadBalancer", "SecurityGroupIDs"]
      }
    }
  }
}

Note: I have given wrong subnet and security group for security reasons. Please check output section there only something I am not doing right