Step function CFT error, Getting error "variable SubnetsParameter in Fn::Sub expression doesn't resolve to string

Below is my statemachine resource template. Failing with template failed variable SubnetsParameter in Fn::Sub expression doesn't resolve to string. Any help would be appreciated.

DefinitionString: !Sub |
  {
    "Comment": "A State Machine to invoke ECS Fargate with Security Groups and Subnet from SSM",
    "StartAt": "RunEcsTask",
    "States": {
      "RunEcsTask": {
        "Type": "Task",
        "Resource": "arn:aws:states:::ecs:runTask.sync",
        "Parameters": {
          "Cluster": "${EcsClusterArn}",
          "TaskDefinition": "${EcsTaskDefinitionArn}",
          "LaunchType": "FARGATE",
          "NetworkConfiguration": {
            "AwsvpcConfiguration": {
              "Subnets": ${SubnetsParameter},
              "SecurityGroups": [
                "${SecurityGroupParameter1}",
                "${SecurityGroupParameter2}",
                "${SecurityGroupParameter3}",
                "${SecurityGroupParameter4}"
              ],
              "AssignPublicIp": "ENABLED"
            }
          },
          "Overrides": {
            "ContainerOverrides": [
              {
                "Name": "${ContainerName}"
              }
            ]
          }
        },
        "End": true
      }
    }
  }