You plan to back up an Azure virtual machine named VM1. You discover that the Backup Pre-Check status displays a status of Warning. What is a possible cause of ...

The Warning state indicates one or more issues in VM's configuration that might lead to backup failures and provides recommended steps to ensure successful backups. Not having the latest VM Agent installed, for example, can cause backups to fail intermittently and falls in this class of issues.
Reference:
https://azure.microsoft.com/en-us/blog/azure-vm-backup-pre-checks/