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.

"properties": {
            "overprovision": true,
            "singlePlacementGroup": true,
            "upgradePolicy": {
                "mode": "Manual"
            },
            "virtualMachineProfile": {
                "networkProfile": {
                    "networkInterfaceConfigurations": [
                        {
                            "name": "testvmss",
                            "properties": {
                                "dnsSettings": {
                                    "dnsServers": []
                                },
                                "enableAcceleratedNetworking": false,
                                "ipConfigurations": [
                                    {
                                        "name": "default",
                                        "properties": {
                                            "privateIPAddressVersion": "IPv4",
                                            "subnet": {
                                                "id": "/subscriptions/xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx/resourceGroup/myResourceGroup/providers/Microsoft.Network/virtualNetworks/testvnet/subnets/testsubnet"
                                            }
                                        }
                                    }
                                ],
                                "primary": true
                            }
                        }