I am getting the following errors when launching an instance using the t2.x types:
400: VPCResourceNotSpecified: The specified instance type can only be used in a VPC. A subnet ID or network interface ID is required to carry out the request.
400: UnsupportedOperation: AMI 'ami-a6386bf4' with an instance-store root device is not supported for the instance type 't2.small'.
400: InvalidParameterCombination: Virtualization type 'hvm' is required for instances of type 't2.small'.
The t2.x instance type is only supported in a VPC environment that is why the first error is thrown when used to launch an instance in a EC2-Classic. And it can only be used with an HVM type image (Hardware-assisted Virtual Machine). This is an AWS limitation.
An example of an HVM image is the following, usually has HVM in the name