The requirements for Proxmox VE are listed below:
 

• The guest VM must be in a running state
   
• The guest VM must not have any snapshots
   
• All of the guest VM's disks must have ample free space available for the internal shadow copy to complete – at least 10% free space should be available
   
   

More detailed information on 'Application-Consistent' backups on Proxmox VE can be found here.
 

To enable 'Application-Consistent' backups on Proxmox VE, check this article.