AWS CloudFormation not creating initial instance in Auto Scaling group

I believe I was able to replicate your symptoms with your template - the issue may be related to the AMI you're using. I selected the AMI for my region from the marketplace link as referenced by the shortened url in the parameter description. I'm still waiting for resource creation to fail (been almost an hour!) but otherwise the symptoms match up.

While launching, the scaling group is visible in the ASG console but with 0 instances. However, the Activity History tab shows a number of attempts to launch an instance, and all fail:

The description on all of them is:

Description: Launching a new EC2 instance. Status Reason: In order to use this AWS Marketplace product you need to accept terms and subscribe. To do so please visit http://aws.amazon.com/marketplace/pp?sku=aw0evgkw8e5c1q413zgy5pjce. Launching EC2 instance failed.

Have you accepted the terms and subscribed to that AMI in the marketplace? There are more details here: https://aws.amazon.com/marketplace/help/200799470#topic1

Note: I believe you will only see the ASG in the console while cloudformation is waiting for it - after the cfn rolls back, the asg (and the activity history) will disappear. As I'm still waiting for the asg resource creation to fail, I haven't definitely confirmed this.

Verify that the subnets where AutoScalingGroup's instances will be installed can connect to Internet using either a NAT gateway or Internet gateway. This because the instances install some packages from Internet in the UserData statement of the stack.