The module azure_rm_virtualmachine has fields for plan information. Module azure_rm_virtualmachinescaleset needs to have them and forward them when creating a VM with an image that requires plan info to be specified. The CIS hardened images from the Azure Marketplace is a prime example.

offer: CoreOS
      publisher: CoreOS
      sku: Stable
      version: latest
    data_disks:
      - lun: 0
        disk_size_gb: 64
        caching: ReadWrite
        managed_disk_type: Standard_LRS
- name: Create a VMSS with a custom image
  azure_rm_virtualmachinescaleset:
    resource_group: myResourceGroup
    name: testvmss
    vm_size: Standard_DS1_v2
    capacity: 2
    virtual_network_name: testvnet
    upgrade_policy: Manual
    subnet_name: testsubnet
    admin_username: adminUser
    admin_password: password01
    managed_disk_type: Standard_LRS
    image: customimage001
- name: Create a VMSS with over 100 instances
  azure_rm_virtualmachinescaleset:
    resource_group: myResourceGroup
    name: testvmss
    vm_size: Standard_DS1_v2
    capacity: 120
    single_placement_group: False
    virtual_network_name: testvnet