Packer Windows Image building: Fast launch setup issue

I am using hashicorp packer to create a windows image with fast launch enabled. When I am enabling fast launch using AWS console, it is getting configured succesfully. But when I am trying to do it using packer build, the process is failing with below error:

Error enabling fast boot for AMI in region us-east-1: retry count exhausted. Last err: InvalidRequest: Can’t enable EC2 Fast Launch. Run instances dry run failed for enabling EC2 Fast Launch. snapshotId cannot be modified on root device.

Any one who can provide some detail or a pointer on this.

This is a scenario where I don’t have a default VPC so I am using Launch template to enable fast launch configuration. The image is getting syspreped successfully before I try to configure it for fast launch. It working through AWS console but not through packer build.