The error is caused by a volume type which has no partition table and due to this is not supported by Veeam Endpoint Backup.
In Windows Disk Management, the volume looks like a common MBR single partition. The Virtual Disk service which is used by Veeam Endpoint Backup to detect disks, generates the following messages that are then forwarded to Veeam log (log location: C:\ProgramData\Veeam\Backup\Endpoint\Backup_Job_*Machine name*\ Job.Backup_Job_Machine_name.Backup.log
Info Layout: MBR, disk signature: 1
Info Partition 0, type 04 (FAT16), offset 0
The MBR partition table occupies sector 1 of the disk, so Partition 0 can start no earlier than sector 2. One sector equals 512 bytes for regular disks and 4096 bytes for 4K native hard disk drives. This means that the offset of Partition 0 must be greater than 512 or 4096.