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.

]
                },
                "osProfile": {
                    "adminUsername": "testuser",
                    "computerNamePrefix": "testvmss",
                    "linuxConfiguration": {
                        "disablePasswordAuthentication": true,
                        "ssh": {
                            "publicKeys": [
                                {
                                    "keyData": "",
                                    "path": "/home/testuser/.ssh/authorized_keys"
                                }
                            ]
                        }
                    },
                    "secrets": []
                },
                "storageProfile": {
                    "dataDisks": [
                        {
                            "caching": "ReadWrite",
                            "createOption": "empty",
                            "diskSizeGB": 64,
                            "lun": 0,
                            "managedDisk": {
                                "storageAccountType": "Standard_LRS"
                            }
                        }
                    ],