cloudformation script fails when invoked via lambda

i have a cloudformation stack file on s3 which works just fine when I launch it from console. but when I trigger the same script using boto3 from AWS lambda, i get an encoded error message. Decoding it then gives me the following json output. (converted to yaml for readability)

---
allowed: false
explicitDeny: false
matchedStatements:
  items: []
failures:
  items: []
context:
  principal:
    id: AROA3H7OMTCQKOORZGNNY:Spawn-reward-service
    arn: arn:aws:sts::773057xxxxxx:assumed-role/Spawn-reward-service-role-g0vgzj5h/Spawn-reward-service
  action: ec2:RequestSpotFleet
  resource: arn:aws:ec2:us-east-1:773057xxxxxx:subnet/subnet-e18921ca
  conditions:
    items:
    - key: ec2:Vpc
      values:
        items:
        - value: arn:aws:ec2:us-east-1:773057xxxxxx:vpc/vpc-aa2884c1
    - key: aws:Resource
      values:
        items:
        - value: subnet/subnet-e18921ca
    - key: aws:Account
      values:
        items:
        - value: '773057xxxxxx'
    - key: ec2:AvailabilityZone
      values:
        items:
        - value: us-east-1e
    - key: ec2:ResourceTag/Name
      values:
        items:
        - value: 1e subnet
    - key: ec2:SubnetID
      values:
        items:
        - value: subnet-e18921ca
    - key: aws:Region
      values:
        items:
        - value: us-east-1
    - key: aws:Service
      values:
        items:
        - value: ec2
    - key: aws:Type
      values:
        items:
        - value: subnet
    - key: 773057xxxxxx:Name
      values:
        items:
        - value: 1e subnet
    - key: ec2:Region
      values:
        items:
        - value: us-east-1
    - key: aws:ARN
      values:
        items:
        - value: arn:aws:ec2:us-east-1:773057xxxxxx:subnet/subnet-e18921ca

The Lambda role has AmazonEC2SpotFleetTaggingRole, NetworkAdministrator and AWSCloudFormationFullAccess policies attached. So not sure where the hangup is. Any help appreciated.

Here is the lambda function in full for reference.

import json, boto3

def lambda_handler(event, context):
    cloudformation = boto3.client('cloudformation')
    response = cloudformation.create_stack(
        StackName='yj',
        TemplateURL='https://s3.amazonaws.com/cf-templates-xxxxxxxxxxxx-us-east-1/reward-service-01-spot.yaml',
        Parameters=[
            {
                'ParameterKey': 'InstanceType',
                'ParameterValue': 't2.large'
            }
        ]
    )
    return response