VM agent is unable to communicate with the Azure Backup Service.
Ensure the VM has network connectivity and the VM agent is latest and running. For more information, please refer to http://go.microsoft.com/fwlink/?LinkID=823919
When I have encountered this message previously, one of the following three actions has resolved the error and allowed backups to work again;
1. Connect to the VM and check the "IaasVmProvider" service. This should be running with the startup type set to Automatic and logging on with the local system account. If this is stopped, start the service.
2. Update the agent on the VM by reinstalling the VMAgent binary as outlined at https://docs.microsoft.com/en-gb/azure/backup/backup-azure-vms-troubleshoot#vm-agent. NOTE: Make sure no backups are running on your VM when you do this.
3. Reinstall the VMSnapshot extension by browsing to the VM in the Azure portal and clicking on "Extensions". Uninstall the "VMSnaphot" extension then try to run the backup again. This will reinstall the extension and automatically start the IaasVMProvider service in the OS.